Precious stones and the gems out from them to highlight their brilliance and colour are fascinating objects Celebrated in poetry and art through the ages; worn by beautiful women; epitomizing wealth, luxury, and power; obtained by strenuous labour and transformed by skill and experience, a precious stone is a pure and tangible concentrate ofvalue, which never loses its appeal, whether as an ornamental object, a collector s item, or simply an investment. in the photo: Picture used in De Beers advertising campaign, with the slogan “diamonds are forever. ” Painting detail from Joseph Heintz, “Danae.” Museum of Dijon.
INTRODUCTION
“A precious stone is a small, rare, hard stone which has inherited from Nature the name of beautiful.” Thus Piero Aloisi,in his classic treatise on gems of fifty years ago, quoted Anselm de Boodt, seventeenth century scholar and physician to Emperor Rudolf ll of Hapsburg. While the notion of size is debatable (there is no reason why a large stone should not be precious, the other four characteristics——rarity, hardness, natural origin, and beauty-together with chemical resistance, all constitute an acceptable definition of a precious stone. A natural object (and therefore a mineral), beautiful, rare, hard, and resistant. Let us look briefly at these properties:
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A precious stone should be a mineral, that is, an object formed spontaneously in nature, without human intervention. This property is essential to our definition, because many modern artificial stones are highly prized, and the synthetic varieties are sometimes virtually indistinguishable from the natural ones. Beauty is essentially a subjective concept, even if the appreciation of precious stones is commonly based on objective criteria, above all optical characteristics, such as dispersive power (the so-called play of colours), colour, transparency, and high refractivity. Fiarity too is a criterion which has more to do with the beholder than the beheld. It is connected to that part of human nature that prefers things that are hard to come by, partly to arouse in others a sense of envy. Despite their intrinsic qualities, no one would wear rubies for ornamentation if they were as common as pebbles on the beach.
The remaining two properties-—hardness and chemical resistance——are truly objective because they are physical and chemical. Hardness is fundamental to a precious stone; scratching of the surface or abrasion of the edges would spoil its appearance. Similarly, poor chemical resistance would eventually lead to partial disintegration, depriving the stone of value by destroying its brilliance. A fabric of subjective and objective properties, fact and fantasy, sensations, fashions, superstition, and reality; celebrated by poets and studied by scientists, depicted by artists and worn by the fair sex, symbol of power and wealth, product of the miner’s toil and the craftsman’s skill——a precious stone is all this—-something which has defied the passage of time and will surely continue to fascinate future
generations: a thing of beauty.
Fantastic origins, not to mention magical and medicinal properties, used to be claimed for precious stones; though references were usually prefaced by something like “it is said that,” or “it has been observed that,” making it hard to determine whether or not the authors believed what they were saying. Ice permanently frozen by intense cold (rock crystal); product of the earth’s extreme aridity combined with the sun’s powerful action (hyacinth); lynxes’ urine and birds’ tears (amber): these are just a few of the fantastic notions about the origins of precious stones-——not differing very much, truth to tell, from those once claimed for minerals of all kinds.
lnnumerable magical -and medicinal properties were attributed to precious stones over the centuries: diamond gave immunity to poison and revealed infidelity; amethyst protected against drunkenness; heliotrope stopped nosebleed and conferred invisibility; sapphire enabled the wearer to escape from prison. These are merely a few better known examples, but there are also countless tales of stones with mysterious names, impossible to identify. We may laugh at all this; but are we ourselves innocent of all trace of superstition? It is worth recalling that at the beginning of this century the Hope blue diamond was alleged to have brought death or economic ruin to its possessors.
The nature of crystals
Observation of quartz crystals, which are mainly of hexagonal,prismatic form or of pyrite, which often consists of perfect cubes, reveals an external regularity, which is verifiable experimentally by measuring the dihedral angles. These are mathematically precise and constant. It is not surprising that this regularity is the reflection of a perfectly regular internal order. This internal perfection, brilliantly perceived at the end of the eighteenth century by the French crystallographer Haiiy and verified experimentally some seventy years ago by the German von Laue, using X-rays, is the crystalline state, manifested in a periodic sequence of constituent particles——atoms or ions-—precisely repeated in all dimensions.
Crystalline state is synonymous with solid state, definable not as that which gives actual form and volume—-as taught in schools—-but as possessing a regular arrangement of atoms, repeated in identical fashion about 100 million times per centimeter. This strict repetition is not found, for instance, in lass,where the order is limited to very few atoms, about 10% (1A[angstrom] = 10‘8 cm) in size, with a random, mosaiclike repetition of- differently oriented “tesserae” some 10 million times per centimeter. Clearly the high quality glass that is commonly called crystal is, scientifically speaking, in the vitreous, not the crystalline, state (Fig. 1).
A crystal is, in fact “a homogeneous body in the form of a polyhedron, bounded by spontaneously formed faces, whose character is determined by the nature of the constituent substance.” A crystal must therefore be formed by a substance in the crystalline state, but a crystalline substance does not necessarily appear in the form of crystals. There are, for example, objects of crystalline substance that have not developed plane faces or others, such as cut precious stones, in which facets have been produced artificially. All precious stones are in the crystalline state, with the exception of opal, which is in the vitreous state.
Symmetry
Crystals display a certain degreeof symmetry, depending on the ordered arrangement of their atomic particles. Symmetry operates in_three ways: by reflection, rotation, and inversion. Reflection, exactly like that produced by a mirror, entails the existence of a plane of symmetry, or a plane dividing the crystal into two specularly equal halves. Rotation takes place about an axis. As the object rotates around this axis, it occupies the same position in space every if’ where n is the order of the axis of symmetry, which in crystals can only be 2, 3, 4, or 6. In contrast, polygons display axes of symmetry from 1 (scalene triangle) to 2 (rectangle), to 3 (equilateral triangle) and so on for regular polygons up to infinity for the circle. Inversion takes place in relation to a center of symmetry, or a point on opposite sides of which identical faces and edges occur (Fig. 2).
In crystals, these elements of symmetry can be variously combined. There are in fact thirty-two different possibilites, known as crystal classes. These thirty-two classes can in turn be allocated to seven crystal systems, which bring together classes having certain elements in common. Without entering into a description of the thirty-two classes, it is worth mentioning the seven crystal systems, as their effects on physical properties are important.
Triclinic system: three unequal crystal axes with mutually oblique intersections.
Monoclinic system: three crystal axes of unequal lengths having one of their intersections oblique, the other two intersections being at 90°.
Orthorhombic system: three crystal axes at right angles to each other, all of different lengths; three unlike planes of symmetry meeting at 90°.
Trigonal system: three like planes of symmetry intersecting at angles of 60° in the vertical axis.
Tetragonal system: three crystal axes at right angles to each other; two of them, taken as the horizontal, being equal; the third, the vertical, being longer or
shorter than the other two.
Hexagonal system: four crystal axes, three of which are equal and lie in the horizontal plane making angles of 60° and 120° with each other, while the fourth axis is
vertical and has a different length (shorter or longer) from that of the horizontalaxes.
Cubic system: three crystal axes at right angles to each otherand of equal lengths.
Finally, there is a further, more general subdivision of crystals into three crystal groups: trimetric, dimetric, and monometric,formed respectively by the first three systems, the second three, and the last (cubic system only). The meaning of these groups is implicit in their names: trimetric denotes three measurements or a different arrangement of atoms and therefore a diversity of crystalline torm in the three dimensions; dimetric meaning two measurements or equality in two directions, and monometric equality in all three directions (Fig. 3).
Some stones are twinned, i.e. with two or more crystals which have grown together according to precise rules of orientation.Twinning produces an additional element of symmetry (one two-fold axis) compared with the typical symmetry for the crystal substance in question. Twin crystals, are often, but not always, recognizable by the presence of reentrant dihedral angles, which are impossible in single crystals. In the case of precious stones, twinned forms of corundum are of particular interest, above all in the ruby variety. Theseare very occasionally seen in the shape of an arrow, consisting of two welded individuals, or more often as a series of close striations on the faces of a single crystal, parallel to the twinning plane.
Physical and chemical properties .
Objects in the crystalline state have a different sequence of atoms in the various directions of the crystal, which as a rule also involves a difference in physical and chemical properties, or anisotropy, a word of Greek origin meaning diversity in different directions. Glass, to the contrary, always displays isotropy, or equality in all directions. ” This difference in behaviour can be demonstrated by subjecting a spherical piece of_glass and another of quartz, to two simple experiments. The first, a chemical test, consists of immersing the two items for a few seconds in hydrofluoric acid, which acts as a solvent, and examining their behaviour as they dissolve. The other experiment, concerning a physical property, thermal expansion, can be performed by heating the two spheres. In both cases, the glass will maintain its spherical shape, despite becoming smaller in the first and larger in the second, while the quartz will assume the form of an ellipsoid of rotation, demonstrating that the crystalline substance behaves differently in various directions (Fig. -4).
Density
This property concerns the crystal as a whole, so that direction is unimportant. Density is defined as weight per unit volume and is expressed in g/cm3, the figures for which basically coincide with those for specific gravity (s.g.), represented by a pure number, corresponding to the ratio of the weight of the substance to that of an equal volume of distilled water.
The density can be determined by one of the following methods. The hydrostatic balance uses Archimedes’ principle, according to which a body immersed in a fluid experiences an upward force equal to the weight of the fluid displaced. Accordingly, if the substance is weighed first in air (W1) and then in water (W2), its specific gravity, according to the definition,
will be:
W1
S.g = —————-
W1 – W2
The pycnorneter (from the Greek, measurer of density), is a bottle with a ground glass stopper pierced by a capillary channel (Fig. 5). The procedure consists first of weighing the substance (W1), then the bottle filled with distilled water (W2),and finally, the bottle, containing the substance, after having eliminated the excess water (W3).
The specific gravity will be:
W1
s.g= ——————
W1 + W2 – W3
The comparison with heavy liquids is based on the elementary fact that a body immersed in a fluid floats to the top, sinks or remains in indifferent equilibrium, depending on whether its density is lesser, greater or equal to that of the fluid. This test can be conducted with one of the heavy liquids, so called because they have a much higher specific gravity than that of water. The most commonly used arebromoform (s.g. 2.9), acetylene tetrabromide (3.0), methylene iodide (3.3), and Clerici’s solution (4.2), which is an aqueous solution of tallium malonate and tallium formate. The first three liquids can be diluted with benzene or toluene, the last with water. The density of the liquid (d1) is varied until the body under examination remains in indifferent equilibrium (dx), accordingly:
dx-d1
The problem then is to establish the exact density of the liquid.This is much easier and can be done either by using a pycnometer—-weighing it first empty, then full of water-—or by means of a special Westphal balance (Fig. 6), which is bascally a hydrostatic balance suitable for liquids, in which the equilibrium of a plunger, calibrated with water, is reestablished using a series of weights which slot into the nine notches on the balance arm.
Cleavage
A mineral subjected to powerful mechanical stress can break. The breakage may occur along irregular conchoidal surfaces—-in which case it is an example of fracture—or along planes corresponding to crystal faces, when it is known as cleavage. This particular type of breakage is only found in crystal substances, indicating a clear difference in cohesion in different directions. When cohesion is much the same in all directions, fracture can occur, even in a crystalline substance.
According to the facility with which it occurs, cleavage can be described by adjectives such as perfect, easy, good, or imperfect. lt can also be described in terms of crystal shape, e.g.cubic, octahedral, rhomb dodecahedral, prismatic, rhombohedral, pinacoidal (i.e. along two parallel faces), and so on. Narrow cracks corresponding to incipient cleavage may often be observed in crystal substances. These are known as cleavage traces and always occur parallel to the planes of symmetry (Fig. 7).
Cleavage is a very useful characteristic in precious stones. Not only is it an aid to recognition, but it makes it easier both to fashion the stones (e.g. it facilitates cutting of diamonds) and to guard against breakage by a suitable choice of setting.
Hardness
Hardness is the result of the greater or lesser cohesion of minerals, or the strength of their chemical bonds. It is definable in terms of resistance to external stresses in one direction(scratching), in two (abrasion) or in three (penetration). Crystallography and mineralogy are mainly concerned with the first. Given the difficulty in measuring hardness precisely, it is expressed in terms of an empirical scale consisting of ten sample minerals of increasing hardness, each of which is capable of scratching the preceding mineral, and being scratched by the subsequent one. This is known as Mohs’ scale and consists of the following:
1. talc 2. apatite 9. corundum
3. gypsum 4.orthoclase 10. diamond
5. calcite 6.quartz
4. fluorite 8. topaz
As examples, the first two items can be scratched by a fingernail (and are therefore “soft”), while a steel point, depending on its type, can scratch the following three or four (known as“hard”), so that the remaining four or five must be “very hard.” .Hardness, like optical properties, is one of the most important characteristics of a precious stone. Considerable hardness, in fact, enhances optical features from luster to refraction-—the play of Iight—-since it enables the surfaces to be kept perfectly smooth and the corners clear-cut. Given the importance of this property, the sections on individual stones in this volume have been arranged in descending order -of hardness. Hardness, commonly associated with something that is unbreakable, has nothing to do with lack of brittleness. Brittleness, or the tendency to break easily, is really related to cleavage—the tendency to” break along precise crystallographic planes. Diamond, the hardest material, is in fact quite brittle, owing to its easy octahedral cleavage.
Optical properties ~
When a light ray encounters a surface separating two different media, such as air and a mineral,-part is reflected, or sent back into the first medium, part is refracted, entering the second medium, and part is absorbed (Fig. 8). Depending on the nature of the substance in question, one or other part can prevail. For example, in metals, a high proportion is absorbed (opaque material) and in many cases a high proportion is also reflected, whereas none is refracted. Conversely, in transparent materials, the refracted portion prevails, while the reflected and absorbed portions both vary (slightly if the mineral is colorless, appreciable if it is colored).
Reflection is governed by two very simple laws: (1) The incident ray, the reflected ray, and the perpendicular to the surface lie in the same plane. (2) The angle of incidence i (formed by the incident ray and the perpendicular) is equal to the angle of reflection i(formed by the reflected ray and the perpendicular):
l = I
There are also two laws for refraction. The first is identical to the one indicated for reflection: incident ray, refracted ray, and perpendicular to surface lie in the same plane. The second states that the ratio of the sine of the angle of incidence i to that of the angle of refraction r is a constant, that is:
sin I = n
——–
sin r
If the incident ray comes from a vacuum (in practice, the air), the constant n is defined as the refractive index of the second medium. In refraction, therefore, a deflection takes place due to the reduction in velocity of the light as it enters a*dih‘erent substance from air and it can be demonstrated that the refractive index n corresponds not just to the quotient of the sines of the angles but to the ratio of the velocity of the light in the two media—-the air and the substance under examination. For crystal substances, which have a diverse arrangement of atoms in different directions, the reduction in velocity of the light generally varies with the direction, so that there is more than one refractive index. Thus monometric crystals and vitreous substances have a single index n, but birefringent crystals have a whole series, whose extremes are nw and ne for dimetric crystals and na and fly for trimetric ones.
The paths of the rays in the two media do not change if the light ray passes from the second medium (the mineral) into the first (the air), traveling away from the perpendicular. The maximum possible departure is 90°, at which point the refracted ray becomes parallel to the boundary between the two media. This angle of refraction is matched by an angle of incidence inside the mineral, known as the critical angle, above which refraction can no longer occur. This is known as total internal reflection, because the entire incident ray is reflected by the boundary into the mineral (Fig. 9).
Two very important instruments for gemstone recognition, associated with the phenomenon of total internal reflection, are the total reflection meter and the total refraction meter, both of which consist basically of a glass hemisphere with a known, very high refractive index. The stone to be examined is placed on the smooth, flat surface of the glass, optical contact being maintained with’a drop of highly retracting liquid. In the total reflection meter, the object is lit through the hemisphere. The rays at small angles will be refracted into the stone, but once the critical angle is exceeded, total internal reflection willoccur. By using a rotating eyepiece with a dial, a clear separation may be obsewed between a light zone (for angles of reflection in excess of the critical angle) and a dark one (for smaller angles, which give rise to refraction). In the total refraction meter, the same result is achieved with grazing light.
In this case, the separation between light and da-rk areas occurs for the refracted ray produced by an incident ray of 90°.The refractive indices of crystal substances generally vary with the direction of propagation of the light rays. If one were to take three very thick but transparent crystals-—rock salt (cubicsystem, monometric group), calcite (trigonal, dimetric), and gypsum (monoclinic, trimetric)—and observe a dot drawn on a piece of paper through them, in the first case, one would see asingle dot, in the second two dots, one of which would be still and the.other rotating as the crystal was rotated and in the third case, two dots again, but both rotating as the crystal was rotated. Because one would normallyexpect to see a single, still image, the conclusion to be drawn from this is that monometric crystals, and for that matter glass, behave normally, i.e. an ordinary ray is propagated in them. In dimetric crystals, there is an ordinary ray plus an extraordinary one, which does not follow the normal laws of refraction. When the ordinary ray as a higher refractive index than the extraordinary ray, the crystal is called positive and vice versa. Finally, in trimetric crystals, there are two extraordinary rays. Thus all crystals, with the exception of cubic ones, ‘display the phenomenon of double refraction, the formation of two polarized light waves traveling in different directions, i.e. the production of two rays of polarized light.
Every motorist knows the eyestrain caused by the sun’s glare on a smooth surface, such as a tarmac road. This glare can virtually be eliminated by the use of special “polaroid” glasses. The rays that cause the glare normally consist of light waves free to oscillate in all possible planes, intersecting one another according to their direction of propagation. Polarization curtails this freedom as the light waves are forced to vi-brate in a single plane (Fig. 10 a,b,c).
To understand how polarization works, an analogy can be made with a stick, which can only pass through the bars of a gate if placed parallel to them. The polaroid lenses in glasses may be compared to the gate, in that the minute crystals of which they are composed, all equally oriented, only let through light rays oscillating in one direction (that of the “bars”), blocking the ones perpendicular to that direction.
Returning to the different behavior of different crystals, we may conclude by saying that monometric crystals and glass are singly refractive (a single ray, consisting of a single light. The phenomenon of tight dispersion; when a light ray passes through a prism, it breaks up into its constituent colors, from red to violet. wave is propagated in all directions) and therefore they have just one refractive index. The other crystals are doubly refractive or birefringent. A light ray entering these crystals is, generally speaking, split into two polarized light waves, each with its own refractive index. The exception occurs when the light enters the crystal in particular directions, called optic axes, at which time the crystals are only singly refractive. The significance of the crystal groups is further demonstrated by the fact that dimetric crystals have only one optic axis, whereas trime trio crystals have two.
It is evident, therefore, that a stone can be assigned to one or other crystal group and, in some cases, even to a crystal system (two valuable aids to recognition) by its behavior in relation to polarized light. This behavior can be analyzed by using a polarizing microscope, or one with two polarizing filters in which the “bars” or directions of vibration are set crossways to each other, and placed one above and one below the rotating stage. Under these conditions, light cannot pass through the microscope, because the light waves coming from the first filter are blocked by the second. The same thing happens if a monometric or vitreous stone is placed on the stage. But if a dimetric or trimetric substance, i.e. one that‘ is birefringent, is examined, it will appear lit up on a dark ground, and then, as the stage of the microscope is rotated, merge with the background in four positions. These are the positions of extinction, in which light cannot pass, due to the coincidence of the directions of vibration of the polarizing filters with those of the birefringent crystal, which can thus be identified. In these positions of extinction, it is possible, keeping only the polarizing filter beneath the stage, to determine the refractive indices for the crystal in that position.
Color and dispersion
Color is extremely important in gemology. A stone looks colored because it absorbs a greater or lesser portion of the rays of the visible spectrum that constitutes white light. Thus a red stone appears red because it absorbs part of the green radiation, and so on. This absorption can affect wide bands of the visible spectrum, or be confined to just a few, corresponding to precise radiations generated by particular types of atoms in the stone. For example, when a sodium chloride crystalis held over a gas flame or more simply, when a drop of salt water boils over from a pan onto a gas ring, the flame turns bright yellow, corresponding to a precise band of the visible spectrum characteristic of sodium.
A given color can be produced in various ways, even by wavebands in different parts of the visible spectrum. It is this very diversity that makes it possible in some cases to distinguish between different stones of similar color and in a few, rare instances, even between natural and synthetic stones of the same type. This is done with an instrument called a spectroscope, which is basically a prism that separates out the colorsof the spectrum, emphasizing the bands of absorption, which look black against the colored background.
A distinctive type of coloration, produced virtually only by opal, is due to the diffraction of light. This physical phenomenon occurs when a light ray encounters a material possessing an ordered internal structure, with intervals of the same order of magnitude as its own wavelength. This might happen with objects shaped like a comb or a row of identical balls, the teeth of the comb or the balls being spaced about ‘/1000 mm, or one micron, apart. Under these conditions, rays are deflected in various directions in relation to the incident ray and these will appear colored due to the disappearance of some radiations by mutual annihilation.
Light absorption can vary enormously according to the nature and thickness of the substance. A substance which, even for minimal thicknesses, absorbs the light completely is called opaque, whereas one that lets through nearly all the light even at considerable thicknesses is called transparent. Between these two extremes there are, of course, various intermediate types of stone, generally described as translucent, because they allow light to penetrate, though not sufficiently for the outline of an object to be distinguished through them. Different behavior in terms of light absorption basically depends on the nature of the substance; thus metals are wholly opaque, as are sulphides and some oxides, while the remaining minerals are generally transparent, above all in single crystals. In fact, one of the most common causesof lack of transparency, apart from the presence of inclusions and minute cracks, is the aggregation of tiny crystals.
Apart from varying in quantity, light absorption can vary in quality, because of the different radiations of the visible spectrum. A substance may appear colorless if there is modest absorption, equal for all colors, but if the absorption is appreciable and affects some colors in particular, the substance will assume a color depending on the mixture of light rays not absorbed. .
Light absorption, like refraction, varies in crystals according to direction. Thus, dimetric and trimetric colored crystals exhibit a greater or lesser degree of pleochroism, i.e. display more than one color due to the different absorption of light in different directions. This phenomenon, which does not apply to monometric crystals and glass, is easily observable under a type of polarizing microscope known as a Haidinger dichroscope. It is possible in this way to observe separately the two colors of a section of pleochroic crystal which, viewed with the naked eye, would merely reveal a mixture of colors. The refractive index is different for each of the colored radiations which, when superimposed, form white light. Thus, when a beam of white light from the air enters the mineral, it breaks up into many colored rays, with different angles of refraction.
This phenomenon is called dispersion of the refractive indices or simply dispersion, and can readily be observed in a transparent substance with nonparallel corners which, when illuminated, produces the sequence of colors of the rainbow: red; orange; yellow; green; blue; violet. The degree of dispersion varies a great deal from one substance to another, as do the mean refractive indices. Dispersion is commonly expressed in terms of the difference in refractive indices for violet and red:
nv- nr
The luster a stone can acquire is also very important to its value. This property depends both on objective criteria, such as the amount of light reflected, and subjective ones, such as the sensation of warmth or coldness it produces. Luster is commonly indicated by a set of adjectives associated with familiar substances:
adamantine diamond, zircon
vitreous ruby, emerald, -quartz
waxy turquoise
pearly moonstone
silky gypsum
metallic hematite
Greasy and dull are also sometimes used to describe luster. The type of luster is~obviously due to the nature of the stone, but its degree is related to surface polish which, of course, is greater the harder the material. The play of light and color is one of the most important qualities of colorless or faintly colored stones. This sparkle of colors is seen mainly in cut diamonds, where the artificial formation of many facets, combined with a very high degree of dispersion, produces a distinct separation of the various colors of the visible spectrum, as a result of a series of refractions inside the stone. Chatoyancy, or the cat’s-eye effect, is due to the presence of minute inclusions of fibrous minerals, such as asbestos, but also to the existence of infinitesimal channels. When the filiform inclusions are so oriented that they are parallel to morethan one crystal face, asterism, a four- or six-pointed star effect, is produced; this is shown to advantage by the cabochon cut. Labradorescence is due to a mosaiclike arrangement of minute tesserae of different compositions, typical of the stone called labradorite, and consisting of a distinctive type of blue green iridescence.
Chemical properties
The chemical properties of precious stones are generally less important to the gemologist than their physical properties. As already mentioned, it is essential that precious stones should be resistant to chemical attack. Tests to determine the chemical properties of stones are not commonly used for purposesof recognition, for the simple reason that any such tests are destructive to the gemstone.Chemical structure can, however, be of interest, and not only for laboratories wishing to reproduce the stones artificially.
Many stones have very simple compositions; e.g. one element (carbon for diamond) or oxides (of aluminum for ruby and sapphire, iron for hematite, silicon for quartz and its different varieties); others are quite complex, containing silicates (emerald, zircon, topaz, garnets) or phosphates (turquoise). Thischemical diversity has completely refuted old ideas about precious stones being similar in composition to one another, but quite different from other minerals because of their physical properties.
For the sake of completeness, it is worth mentioning two some what opposed phenomena relating to the physical and chemical properties: isomorphism and polymorphism. Polymorphism(from the Greek, many forms) applies to substances that develop different crystal structures according to their environment (temperature, pressure, chemical environment), with the result that their external appearance and properties differ, sometimes considerably. Carbon, for example, exists in nature as diamond, cubic, colorless, transparent, and very hard (H=10), or-graphite, hexagonal, blackish, opaque, and very soft (H=1). Obviously only the diamond form is used as a precious stone or a very high-quality abrasive, while graphite has completely different applications, exploiting its specific properties such as very low hardness combined with color (pencil) or very good electrical conductivity and chemical resistance (electrodes for chemical processes). Some polymorphous forms are stable under certain environmental, conditions, others are always unstable and therefore tend to transform themselves into the stable varieties. It is very important to establish, where possible, which are the correct environmental conditions for the various forms of a polymorphous substance,not just out of scientific interest, but for the practical -effects this can have on possible synthesis of the form desired (for example, diamond from graphite). This phenomenon involves different crystal structures, as distinct from different varieties of the same crystal type, e.g. ruby and sapphire, which differ in color but have the same structure and chemical composition——those of the mineral corundum. isomorphism (from the Greek, same form) occurs when two or more chemical substances have identical crystal structures and are chemically so alike that they can form solid solutions,or in other words, mixed, homogeneous crystals in which thecorresponding atoms change places at random and in varying proportions. Examples of interest to gemologists are garnets and olivines. Take, for example, -pyrope Mg3Al2Si3O,2 and almandine Fe3Al2Si3O,2 in the case of garnets, and forsterite Mg2SiO4 and fayalite Fe2SiO4, the two basic constituents of olivine. In both instances, the close crystallochemical resem blance between magnesium (Mg) and iron (Fe) causes these atoms—or rather ions—to change places at random, acting like a single chemical element in the crystal structure. In fact,nearly all pyropes can be given the formula (Mg, Fe)3Al2Si3O,2 and periolots the general formula (Mg, Fe)2SiO4. These are known as crystallochemical formulae, the comma in parentheses signifying “or” and denoting a substitution, according tothe following, straightforward chemical ratio Mg+Fe:Si:0=2:1 :4. More complex cases can involve the simultaneous substitution of several atoms, even by groups of different elements. Examples of this are pyroxenes (including jadeite) or plagioclases (e.g. sunstone and labradorite). With the latter in particular-, sodium (Na) is replaced by calcium (Ca) at the same time as silicon (Si) is replaced by aluminum (Al), thus their crystallochemical formula can be written NaAlSi3O8=CaAl2Si208.
Crystal structure
As is well known, X rays are very penetrating radiations which are differentially absorbed by various substances. Radiology, which is mainly practiced on the human body for diagnostic purposes, is based on this principle. Slight individual differences in chemical composition or in thickness within the body show up as different shadows on a photographic plate. The same thing happens with crystal substances which, however, also display another, more complex phenomenon known as diffraction.
A crystal struck by an X ray, apart from causinga more or less marked reduction in intensity of the incident ray passing through it, gives off a series of deflected rays, called diffracted rays, the direction and intensity of which depend on the reciprocal arrangement of its constituent atoms. These atoms form a three-dimensional motif which is repeated millions of times in identical fashion throughout the crystal; this motif is known as the crystal structure.
The crystal structure of different substances is established by highly specialized scientists called crystallographers, who make complex calculations after examining the diffracted rays.Such calculations are of -little concern“ to the general public, or even the majority of students‘, who are interested, at most, in the effects such structures have on the properties of s_ubstances.X-ray diffraction, however, is very commonly used for recognition of various crystal substances, because the pattern of diffracted rays is specific to each substance, virtually like a “fingerprint.” Although by far the most common means of identification in mineralogy, it is hardly ever used in gemology, for two reasons. The first is that the easiest and best technique requires dust specimens of the substance in question, obtainable by grinding, which is -clearly inappropriate for precious stones. The second problem with X-ray diffraction techniques is that X ray interaction with substances within the specimen can cause fluorescence, which may permanently alter its color, On the other hand, such variations in color are sometimes deliberately produced in laboratories, to simulate the more valuable varieties of natural stones.
Genesis
The processes whereby minerals are formed are related to the origins of rocks in general, which are in fact associations of minerals. For the-sake of convenience, rocks are divided into three groups corresponding to the way in which they were formed, magmatic (or igneous), metamorphic, and sedimentary, although the different categories inevitably overlap.
The magmatic or igneous process depends on the existence of original magma, a molten silicate mass containing many volatile compounds in solution (water, hydrochloric acid, hydrofluoric acid, carbon dioxide, etc.). This molten mass, which is presumed to exist in certain areas within the earth’s crust, can rise to the surface through volcanic pipes to form lava, releasing its volatile components into the atmosphere. But in the absence of a passage to the outside, the magmatic mass cools very slowly, leading to the formation of crystalline rocks (at temperatures in the order of 800°-1 000° C) “similar in compo sition to lava, but with the constituent minerals present as larger crystals. The volatile components, unable to escape into the atmosphere, tend to accumulate, increasing in quantity due to crystallization. This growth causes a powerful increase in pressure, resulting in the formation of a fluid with particular characteristics; it is mobile like gas but dense like water, and can infiltrate the surrounding rocks,’often making chemical exchanges with them. As the pegmatitic fluid cools, crystals of large, sometimes gigantic proportions, are deposited. With progressive cooling, this phase passes into hot water or hydrothermal phases containing many chemical constituents in solution, which will be deposited during the cooling process, sometimes forming fine crystals. To make a simple analogy, the magma can be compared to soda pop in a bottle. If the cork is removed (the volcanic pipe opened up), there will be a rapid discharge of gas, both as such and combined with the liquid as foam. If, on the other hand, the bottle is placed,unopened, in a freezer (slow cooling of the magma inside the earth’s crust), the water will turn to ice, unable to dissolve the gas, which will collect near the cork with a buildup in pressure.
The metamorphic process is related to an increase in temperature (from 300° to 600° C), often combined with an increase in pressure, to which existing rocks that are formed atdifferent (greater or lesser) temperatures may be subjected.
This increase in temperature can be due to proximity of a magmatic mass of very high temperature but also-—and this is more usually the case-—to the rock’s sinking in the earth’s crust, obviously leading to an increase in pressure. These changing environmental conditions, sometimes combined with circulation of fluids of particular chemical compositions, produce a rearrangement of the mineralogical groups, and the formation of new minerals, characteristic of these new conditions. An everyday comparison to illustrate the process is a flour and water dough which, baked in the oven, turns to bread.
The third process is the sedimentary process, the easiest to understand because it takes‘ place under our very eyes. Existing rocks are not stable in our environment, which is characterized by low temperatures (from -50° C to +50° C), low pressures, and a plentiful supply of water and oxygen. The rocks physically disintegrate into boulders, pebbles, and sand and undergo chemical changes as well, some components dissolving in water, other, more resistant minerals remaining unaltered. The parts in solution can be deposited at various stages of their journey (by streams, rivers, the sea). Thus, the crumbling of a loosely compacted stone into sand, the deposition of travertine by a calcareous spring, and the crystallization of rock salt in a pool on the seashore are all part of the sedimentary process.
Any of these processes, which together roughly account for all the rocks of the earth’s crust, can lead to the formation of precious stones, many of which are attributable to more than one such process. For example, because of their hardness and chemical resistance, precious stones are nearly always found in sedimentary deposits. These are known as secondary deposits because they are derived, through the physical disintegration and chemical alteration of the country rock, from primary deposits where they were first formed by crystallization from magma, pegmatitic fluid or aqueous solutions, or by metamorphic recrystallization.
It is well worth knowing the origins of stones not just out of scientific interest, but for practical reasons as well, because any attempt at synthesis of gemstones should obviously aim to reproduce in the laboratory the conditions which led to their formation in nature. Clearly, however, such conditions can only be approximated, partly because natural processes are always much more complex than artificial ones, and mainly because of the vast time-scale at Nature’s disposal. On the other hand, as we shall see in due course, man, apart from attempting the synthetic reproduction of precious stones, can go Nature one better by producing in the laboratory precious artificial stones_ not found naturally because they are made up of chemical components that are rarely available in sufficient quantities or are outnumbered by other elements.
Units of measurement
The common unit of measurement for precious stones is weight, expressed in carats and decimal parts thereof. A carat is equal to 0.200 g, a standard value fixed at the end of the nineteenth century to unify the traditional values of individual markets (in Italy, for example, they varied from 0.188 g in Bologna to 0.216 g in Livorno). Similarly, subdivision of the carat into a hundred points has replaced the old binary system according to which a grain was worth a quarter of a carat.
The reference to a grain of wheat seems clear enough, but the derivation of the word carat is less certain. lt probably came from the carob seed (qirat in Arabic) which, being quite constant in weight, once served as a counterweight on the market for precious stones.
Some confusion is created by the system of subdivision into carats that is applied to gold-working. This referred to the ownership of a merchant vessel, which was traditionally divided into twenty-four shares. Thus pure gold is 24 carat, or 24k, the gold commonly used in jewelry is 18 carat, or an alloy which is ‘8/24 or 75 percent gold, and so on. Diameter measurements expressed in millimeters and fractions thereof are sometimes used instead of carat weights for synthetic stones of modest value and for necklace beads.
Methods of analysis
Probably the first question one tends to ask sbout a gem, as of a mineral, is “What is it?” Some look quite similar, so much so that before their chemical composition was known (i.e. before the eighteenth century) they were easily confused and given the same names. The answer to the question “What is it?“ is primarily mineralogical, if, for example, a ruby needs to be distinguished from a garnet or sp_inel of similar color but, among other things, of different value, or a green tourmaline from an emerald; but it is also gemological and commercial, whenever a natural ruby or sapphire has to be distinguished from its synthetic counterparts or from doublets which, as we shall see, are particular types of imitation.
Chemical methods are not normally employed to answer this question, as they involve the destruction of at least part of the object, and of the physical methods, only those forms are used that do not cause appreciable changes or, at any rate, damage to the gems in question.
Establishment of the density
One of the basic physical properties used for identification of precious stones, and one of the easiest to establish, is density. As mentioned earlier, two main -methods are employed to determine this: heavy liquids (the two variations of which we shall examine) and the hydrostatic balance. The first method in its simplest version, most often used with gems, calls for a limited number of glass bottles containing liquids of known densities, which form a scale. As each bottle normally contains a mixture of two chemicals, one of which may be volatile, a fragment of a mineral of known density, appropriate to that of the liquid, is kept in each bottle by way of control. If the liquid has the proper proportion of each of its constituents, the mineral should remain in indifferent equilibrium. A suitable series of liquids that can be bought already prepared is shown in the table opposite, along with the respective indicators (Fig. 11).
It should be noted that these chemicals can be extremely dangerous. Anyone using them should take appropriate precautions, including the use of a chemical hood and gloves.
If a gem under examination is put, for example, into bottle No.2, one will note: d>2.71 g/cm3 if it sinks; d2.71 g/cma. One tries bottle No. 3, which gives the result d
Where a more precise figure is required, two pairs of liquids are used: methylene iodide diluted with toluene, and Clerici’s solution diluted with water. Each pair of liquids can be mixed in variable proportions until it equals the density of the object under examination. The first pair cover a range of 2.00 to 3.30 g/cma and the second pair a range of 3.00 to 4.02 g/cms.
Each of these mixtures shows a linear relationship between density and refractive index, as can be seen from the graphs in Figures 12 and 13. Therefore, if the refractive index of a drop of liquid from a mixture prepared to equal the density of a given gem is measured with an ordinary jeweler’s refractometer, one can determine the density of the liquid and consequently of the gem remaining in indifferent equilibrium in it.
Failing a refractometer, a pycnometer or balance can be used to determine the density of the liquid, as noted previously.
The heavy liquid method (especially the first version) is quite straightforward and works very well even with stones of very small volume, but the abovementioned liquids, i.e. the ones that are easiest to use, cannot determine densities in excess of about 4.10 g/cma. The hydrostatic balance method can be used whenever a reasonably precise balance is available, as is generally the case where gems are dealt with. To transform the balance into a hydrostatic balance, all that need be done is to place a “bridge” with a transparent container of distilled water on topof it over one of the pans. A small wire basket to contain the gem is suspended from the arm of the balance by a thin piece of wire, in such a way that it is fully immersed in the water (Fig. 14).
The specific gravity of a stone, which is also its density, equals the weight of the stone divided by the weight of an equal volume of distilled water. To obtain this figure, the following procedure is followed: the stone under examination is weighted in air (W1), and then, using the apparatus described above, it is weighed immersed in distilled water (W2); the difference between the two weights (W1—W2) represents the loss of weight due to Archimedes’ principle and this, assuming that distilled water always has a density equal to that of 1 g/cm?’ (which is not strictly correct, but is near enough for our purposes), corresponds numerically to the volume of the stone. Thus, using
the formula sp. g.r. = W1
—————–
W1 – W2
one can calculate the specific gravity of the stone.
The chief complication is due to the fact that the weight of the stone in immersion must, in turn, be calculated as the difference between two weights: that is, one must read how much the balance registers with the equipment for specific gravity measurement and the basket immersed in water, then how much it registers with the stone in the basket. The difference is the weight of the stone in immersion.
With the hydrostatic balance, one can determine the specific gravity of a gem even when it is more than 4, provided the gem is not of too limited weight or rather, too small volume. In the case of artificial diamond simulants, for example, which have a specific gravity of between 5 and 7, or thereabouts, it is difficult to obtain acceptable results with stones of less than 0.5-1 carat, depending on the precision of the balance. The necessary Iack of absolute precision is not usually a serious problem with gemstones, as one is concerned with no more than a hundred different minerals at the outside, of which only about twenty are very common.
Establishment of the optical properties
Certain optical properties are a valuable. aid to identification and are readily established by means of a gemological instrument called a refractometer (Fig. 15“), which is a simplified version of the total refraction meters used in mineralogy. With this instrument, if a yellow filter is placed over the eyepiece or a sodium light employed, the refractive index (or indices in the case of a doubly refractive mineral) can be read straight off, as can the value for birefringence of which the optic sign can also be established. In this way, for example, in the case of a red stone with a specific gravity of 4.0, one can readily distinguish between a negative, uniaxial, birefringent ruby corundum with indices of about 1 .761-1.770 and birefringence of 0.009, and a garnet of the pyrope-almandine series, which has only one index, of between 1 .775 and 1 .790 when the specific gravity is 4.0, proving that it is singly refractive, with an index just enough higher to make a distinction possible (see final synoptic tables).
It is important when using a refractometer for the specimen to have a flat, polished surface. This is always possible in the case of gems. The reading is taken by placing the refractometer in front of any type of strong lamp, with a yellow filter fitted over the eyepiece, or in front of a sodium discharge lamp in the absence of a filter. A drop of special contact liquid is put on the prism of the refractometer, one facet of the gem under examination being placed on top of this. The calibrated scale is visible through the eyepiece (Fig. 16). The point at which the dividing line between the areas of light and shade falls on the graduated scale indicates the refractive index, or, where two lines are visible, indices. The stone is turned on the refractometer glass, and the values of the two indices are read when the greatest distance between the lines is achieved. This maximum distance is the value of_ the stone’s birefringence.
In the case of a uniaxial, birefringent gem, one of the two shadows (that of the ordinary ray) will remain stationary during rotation, while the other will move away from it and then return. The crystal is optically negative if the index of the extraordinary ray is lower than that of the ordinary ray and optically positive if the opposite is the case. In biaxial, birefringent gems, both shadow edges, marking the two indices, are seen to move, and the reading is given by the top figure legible for the higher one and the bottom figure legible for the lower one.
The maximum birefringence is indicated by the difference between the two indices thus established. One need not usually be concerned with the optic sign (positive or negative), or with uniaxial, birefringent stones.
In the case of singly refractive stones, everything is much simpler, as only one shadow edge is visible (Fig. 16a); this edge remains stationary when the gem is rotated. With the refractometer, one can measure refractive indices of between 1.40 and 1.80. All told, this excludes diamond, zircon, some garnets, rutile, some synthetic diamond simulants and a few rarities, for which other methods have to be used.
But the inability of the refractometer to measure the refractive index of a given gem is in itself a clue to identification, corresponding to n>c.1.80, which greatly reduces the area of uncertainty.
There are various other ways of determining the approximate refractive index of a gem and whether or not it is birefringent, but only the simplest ones are listed here. These are based on elementary observations not calling for any special equipment. Obvious pleochroism is always a sign of birefringence. But the opposite is not the case: absence of clear pleochroism does not mean that a stone is singly refractive. Pleochroism is particularly evident in some sapphires and rubies, in tanzanite, cordierite, intensely colored tourmalines, andalusite, and aquamarine; it is very useful for rapid identification of these gems. Marked birefringence can be established by looking through a stone from a flat facet with a 10x lens. The birefringence will be manifested by a double image of .the opposite facet edges. This phenomenon is easily detectable in zircons, peridot, and synthetic rutile; a bit less so in tourmaline, kunzite, hiddenite, and diopside. It can also be visible in quartz corundum, and topaz, but only in fairly large stones. This is a very simple method, for example, of distinguishing a singly refractive diamond from a strongly birefringent colorless zircon,or synthetic rutile, without further investigation. It also makes it possible to distinguish zircons of various colors from other stones, except peridot.
One means of judging the approximate refractive index of a gem is to observe it in transmitted light, immersed in a highly retracting liquid in a transparent container. If the refractive index of the stone is equal to that of the liquid, it will be barely, if at all, distinguishable from the surrounding liquid. If the index of the stone is higher than that of the liquid, it will be clearly visible, standing out the more clearly, with dark edges, the stronger the difference in index. This method can be useful if there is no flat facet to permit reading of the index by means of a refractometer. A complete series of liquids for determination by this method is listed in the table below. But often just two or three liquids with different indices are enough. If necessary, the refractometer can be used to determine the index of the liquid or liquids used.
Spectroscopy
The light transmitted or reflected by a body can be “analyzed” with an instrument consisting basically of a glass prism, by observing which rays are transmitted and which absorbed. The result is what is known as the absorption spectrum of an object, in our case of a gem, and this can to some extent be quantified by superimposing a graduated scale from 400 to 800 nanometers on the path of the light rays.
In idiochromatic minerals, the color of a gem (and its spectrum, with possible areas of absorption at different wavelengths) depends on the basic composition of the mineral. In allochromatic minerals, it depends on the presence of -very small quantities of other, color-producing elements in certain clearly defined structural positions, or simple structural anomalies. In either case, the various minerals generally have characteristic absorption spectra.
Spectroscopy is used less often than measurement of the specific gravity and refractive indices to identify gems, perhaps because it is less intuitive. But it is extremely useful in the case of rough stones without any flat facets suitable for the refractometer, gems with a refractive index in excess of 1.80, those devoid of significant inclusions, and instances where spectroscopic differences can clearly distinguish between a natural gem and its synthetic counterpart. Furthermore, it is normally a very rapid means of identification.
An optical spectroscope, however, only reveals lines or bands of absorption in areas where there is a significant jump in absorption of the respective wavelengths. Areas of weak absorption, with very hazy edges, affect the color of the gem, but are only detectable by apparatus that is more sensitive than the human eye and is equipped with amplification systems. There fore the eye is not capable of detecting the absorption spectrum of all gems, but only those in which it is most pronounced.Conversely, some gems have a clearly delineated spectrum,despite being colorless, because of the complementarity between different zones of absorption.
Two of the most typical spectra are shown in Figs. 17a and 17b; they are those of zircon and green sapphire, respectively. The stones can look very similar externally but have clearly different absorption spectra. Figs. 17c and 1 7d give the spectrafor ruby and almandine garnet, which are also of similar color.
Here, too, rapid inspection can immediately distinguish them, given the marked difference in their absorption spectra.
Observation under a binocular microscope
A lot of information useful for gemstone recognition can be obtained by meticulous observation under a stereoscopic microscope of low magnification (from 10x to 40x).
The illumination of the object observed is very important. The possible methods of illumination are:
Transmitted light, with a light source beneath the stage of the microscope, preferably capable of producing polarized light;
“dark field” illumination, in which the light reaches the object laterally, from all sides, in such a way that the only light entering the microscope is that sent back to it by the inclusions contained in the object, which stand out like dust lit up in a dark room by a ray of sunlight;
reflected light coming laterally from above, for instance, by placing the stone in a container of highly retracting liquid, normally monobromonaphthalene (n = 1.658). Immersion cancels out the reflections produced by the upper facets, which would make the internal details hard to see.
Observations carried out in this manner can reveal certain important aspects of the specimen:
Crystallography-—CrystaIlographic striae which are faint or emphasized by bands of color or areas of inclusions can give an idea of the crystal system to which the stone belongs. Sometimes, multiple twin structures are also visible, and these are particularly evident in polarized light. They are frequent in rubies, but also in some sapphires.
Crystallographic optics——One can detect birefringence or areas of anomalous birefringence in a singly refractive crystal, or pleochroism, which is also a sign of birefringence. If stones are observed inimmersion, one can also obtain approximate information on the refractive index, by comparison with that of the liquid.
lnclusions—Many gems are at least five to six millimeters in size, and often ten or fifteen millimeters or more. Hence they are quite.we|l developed crystals which took a very long time to grow. During this time, other crystals were forming in the same environment, but the ones that began crystallizing first and/ or grew less will sometimes have been incorporated in the larger ones. Residues of the liquids and
gases that constituted the fluid from which the crystals were formed may also have been trapped inside them. These partially crystalline, liquid residues sometimes penetrated a crack in the ready-formed crystal at a later stage, helping to “heal” it to some extent, as long as the necessary material to “rebuild” the broken crystal was present in solution.There can also be inclusions which have separated out from the engulfing mineral during cooling, such as the needlelike crystals of rutile found in many corundums, which separated out from the corundum as the temperature fell.
From the overall picture of inclusions, corresponding to a precise chemical and physical environment of formation, one can often recognize not only the crystal species of a gem containing them, but also, the locality in which it was deposited. Thus the type of inclusions found in a Burmese ruby are generally sufficient to identify it as ruby and distinguish it from, say, a ruby with the inclusions typical of rubies found in Thailand. Both of these, moreover, are usually readily distinguishable, under a microscope, from a red garnet and normally from red tourmaline and red spinel as well.
he same applies to emerald: observation of the inclusions usually makes it possible not only to identify the stone as emerald, but also to distinguish a Colombian gem from a Zimbabwean specimen.
Synthetic products with the same chemical and physical characteristics as those of their natural counterparts can pose problems. But the inclusions and structures visible inside synthetic products are related to a much faster pattern of growth, which is often quite different from the natural one. Thus, observation of inclusions and possible non crystallographic growth lines is the principal means of distinguishing them.
Finally, some natural stones are treated in a variety of ways to heighten their desirable characteristics. These procedures will be discussed in more detail in the descriptions of individual stones. Many of these treatments are clearly fraudulent, but some have been accepted as legitimate. The standard accepted by the jewelry trade is that any treatment that cannot be detected under magnification or through testing and that is irreversible is acceptable. Treatments that can be identified or that will deteriorate over time must be revealed whenever the gem is sold.
Having covered the basic methods used to distinguish precious stones from one another we describe below some minor tests which can sometimes help with identification where the principal procedures are inconclusive or cannot be applied. These are basically hardness tests and observation of fluorescence under ultraviolet light.
Hardness tests
We have already seen briefly what is meant by hardness in mineralogical terms and how it is expressed in terms of the Mohs’ scale, consisting of ten sample minerals arranged in in creasing order of hardness so that each can scratch the preceding one and be scratched by the following one. Hardness tests on cut stones are avoided, whenever possible, in order not to compromise the luster and integrity of the facet edges; however, such problems do not arise when one is dealing with rough stones. In any case, the results cannot be expected to be very significant (except for diamond), because the majority of precious stones have a hardness of between 7 and 9 on Mohs’ scale.
Furthermore, if the tests are carefully performed, in order to leave as little trace as possible, they are hard to interpret. For this reason, they are nearly always done under a binocular microscope, using a series of points consisting of very small fragments of minerals from Mohs’ scale, which are inserted into the tip of a metal rod roughly the size and shape of a ball point pen. Starting with a point presumed to be less hard than the object, an attempt is made to scratch the outer edge of one facet, generally near to the girdle (the band at the widest part of the stone) or even on the girdle, if it is not too rough. If the gem cannot be scratched, for example, by a point with a hardness of 7, the conclusion is that H>7. This can be sufficient information if all one needs to know is whether it is a gem with a hardness of 8, or another one like it, with a hardness of 6. lf this is not sufficient, a point with a hardness of 8 can be tested and according to the result one will note H>8 or 7<H-
Examination of fluorescence under ultraviolet light One last, simple test used for gem recognition, particularly when other methods are hard to apply, consists of observing their fluorescence (and possible phosphorescence) by exposing them to ultraviolet light. Fluorescence basically means the reemission of radiations of greater wavelength by an object struck by any type of radiation- In this particular case, we mean the reemission of light waves by a stone subjected to ultraviolet rays. Reemission of light waves after the incident radiation has ceased is known as phosphorescence. Many different types of lamp can be used, but the commonest is one that was invented for mineral prospecting. It has two tubes, with filters to provide radiations with wavelengths of 254 nanometers (SW or short wave) and 365 nanometers (LW or long wave), which can be operated separately by a switch.
Various stones react by displaying quite striking fluorescence, which in some cases can identify them, mainly by a process of elimination. For example, strong red fluorescence on LW and greenish blue fluorescence on SW are seen in light blue synthetic spinels used to imitate aquamarine, which is not fluorescent; ruby and red spinel, which display red fluorescence, can be distinguished from garnet and red tourmaline, which never do (but this test will not distinguish ruby from spinel). The apricot yellow fluorescence of the pale yellow sapphires from SriLanka immediately distinguishes them from the corresponding synthetic product and from citrine quartz. Synthetic blue sapphires have a soft, opaque greenish blue fluorescence (on SW only), which normally distinguishes them from the natural varieties. Kunzite sometimes has pink fluorescence, which readily distinguishes it from morganite and synthetic pink corundum. Finally, opals have characteristic fluorescence andphosphorescence, different from their synthetic counterparts. Such data, however, are qualitative and always require interpretation by the operator, who must avoid hasty conclusions. part from these, which are the most usual tests, various other means of investigation are occasionally used for gemstone recognition. Of these, the reflect meter (an instrument for determining the reflective power of a flat, polished facet) and the thermal conductivity tester are two instruments employed almost exclusively in gemology; but their use is very limited and confined to the distinction of diamond from its imitations.
Cutting of gemstones
Cutting is the operation whereby a rough stone is made to as sume a certain shape, which brings out its luster and color and enables it to be set in an item of jewelry.
In the old days, most precious stones were used with the natural facets of their crystal structure or were summarily rounded and polished into a convex shape known as a cabochon, from caboche, an old Norman French word for head (Fig. 19). This type of cut is now only used for stones of limited transparency, either by nature (e-g. turquoise, jadeite, malachite), or as a result of too many inclusions (e.g. some relatively opaque sapphires, rubies, and emeralds). Or else, it is used for gems in which curved surfaces bring out certain special characteristics (e.g. opal, star sapphires and rubies, adularia, and “chatoyant“ stones).
Over the last 300 years, however, the faceted type of cut has become increasingly well established. This best displays the beauty of transparent stones because of the fact that light is reflected both by the upper facets and, through the stone, by the lower ones. The faceted cut has been most important in highlighting the qualities of diamond, which for a long time was less appreciated than other gems and only used when found as natural, octahedral crystals. Faceting could display diamond’s extraordinary ability to reflect light, even breaking it up into the colors of the rainbow, because of its high power of dispersion.
The earliest record of cut diamonds dates from the mid-fourteenth century. Initially, cutting probably only served to enable rough diamonds of an unattractive, irregular shape to be used as gems, and involved the creation of one or more series of facets arranged in a radiating pattern around the gem, the base of which was left uncut or reasonably flat. Some famous Indian diamonds were also cut into these radiating shapes, known as rose or rosette cuts, only used nowadays for small stones (Fig. 20).
Subsequently, around the middle of the fifteenth century, the shape of octahedral diamonds also began to be modified, creating a flat, upper facet instead of a point and smoothing a few corners into other, flat facets, in an effort to make the stones more brilliant (Figs. 21, 22, 23). When it was realized that the angles of the octahedron were not ideal for producing light reflection and refraction, the cut was modified in such a way that only the general outline of the original octahedron was left. By the early eighteenth century, diamonds were already being given what is known as an old mine cut with facets corresponding to those of the modern brilliant cut (Fig. 24), with a more or less octagonal table facet on top, 4 main facets and 4 corner facets, plus 8 star facets and 8 pairs of cross facets, connecting the table to what is known as the girdle to form the crown. The girdle was almost square (Fig. 24), or almost rectangular (Fig. 25) in shape, with convex sides and rounded corners (cushion-shaped, not circular, as it is today). The lower portion, called the pavilion, terminated by a small facet parallel to the first, had 4 main facets plus 4 corner facets and 8 pairs of cross facets opposite the crown facets.
Only later was the practice established of turning stones on a lathe, using another diamond, to give them circular symmetry before faceting. In this way, the corner facets became similar in appearance to the main ones. Finally, at the beginning of the twentieth century, the brilliant cut was given its present shape and proportions, the facets being of the number and arrangement already described, but angled in such a way as to obtain the maximum degree of light reflection and dispersion (Fig. 26). Nowadays, most diamonds are in fact given a brilliant cut. Although the proportions of the cut stones are usually not ideal, they depart very little from the norm. The (round) brilliant cut is the most common, but diamonds are also cut into oval, pear or marquise shapes, in which the number and shape of the facets is the same as those used for the brilliant cut (Fig.28). Alternatively, some diamonds are cut into rectangular shape with truncated corners, and a crown and pavilion consisting of successive series of trapezoidal facets. This is known as the step, trap, or “emerald” cut (Fig. 29). It gives a less lustrous effect, with lower dispersion, but reduces weight loss when the uncut crystal is in the shape of an elongated octahedron. The problem of weight loss during cutting is very important, given the rarity and value of diamond: as a rule, a brilliant-cut stone weighs only 40 percent or so of the original rough stone.
When the faceted cut is used for stones other than diamond, the rules followed are less precise. The main aim is normally to reduce loss of weight as much as possible, and in colored stones with distinct pleochroism to have the most attractive color visible from the table facet.
In most faceted stones (as in Fig. 24), one can distinguish the main, table facet, which is generally larger than the others, and forms the topmost part of the stone; the crown, consisting of numerous facets linking the table to the girdle; the girdle, which is the band at the widest part of the stone, onto which a setting can be fitted; and the pavilion, which is the lower, convex portion, of roughly conical shape, sometimes terminated by a bottom facet (the “cutlet”) parallel to the main one, but generally much smaller. Depending on the type and arrangement of the facets, the most common cuts are as follows: ‘ brilliant, when the shape and number of facets are as prescribed; – step or trap, if both the crown and pavilion consist of successive series of trapezoidal facets (the “steps”); 1 mixed, if they have a more or less brilliant-cut crown and step-cut pavilion.
Depending on their general outline, they may be round, oval, marquise, pear-shaped, rectangular, emerald (rectangular with truncated corners), cushion (vaguely rectangular, but with curved sides), triangular, and even star-shaped. The most common shapes are shown on pages 69-72 (Figs. 28-31) The methods used for cutting stones have not changed essentially over the centuries, although the details have been greatly improved. The normally quite small stones are fixed to the tip of a mushroom-shaped support called a dop stick, by means of a very strong cement or low melting point solder. Nowadays, dops are also made with a special clamp that grips the stone firmly, but allows just enough play for cutting operations. For the cabochon cut, grinding machines not very different from the types used for sharpening knives are employed, firstly with a coarser grain to remove any rough surfaces and give the future gem its shape, then with a finer one to polish the surfaces. The final polishing is usually done on a felt, leather, or fabric covered revolving horizontal lap, onto which very fine, abrasive powder has been sprinkled.
The cutting of faceted stones is more complex and is carried out in several stages. A revolving horizontal metal lap dressed with a paste of very fine abrasive powder plus oil or water, is used for grinding.
For diamonds, if the rough stone is fairly irregular in shape, advantage is taken of its easy cleavage to reduce it to the typical octahedral form or, at any rate, obtain more easily workable pieces. lf the rough stone is already octahedral, part of one pyramid is removed (Fig. 27a), using a sawing discharged with diamond powder in oil, or by working it against another diamond, in the position where the table facet will be produced (Fig. 27b). It is then rotated on a lathe, bringing it into contact with another diamond, to give it a conical-cylindrical shape (Fig. 27b, c). These two stages are collectively known as “bruting.”
The facets are then cut on a cast iron lap dressed with a mixture of diamond dust and oil (because of its exceptional hardness, diamond is only appreciably abraded by another diamond), starting with the 4 main top facets for the crown and the 4 main back facets for the pavilion (Fig. 27d, e). The exact position of the first 4+4 main facets is very important, and this stage is carried out by highly skilled personnel. The subsequent facets (Fig. 27f, g) are less difficult to produce. This stage constitutes the cutting proper. The cutting and polishing of the facets can be done in a single operation, or in two stages. In the latter case, a slightly coarser diamond powder isused to begin with, to save time, then a finer one, for polishing.
With gems other than diamonds, if the rough stone is large and irregular in shape, it is first sawn into pieces .of a suitable size for cutting, although few precious stones (topaz and spodumene are among them) have strong enough cleavage for this to be done advantageously. The operation, which also serves to eliminate any badly flawed areas of the rough stone, can also be done with a small hammer—wielded, of course, with suitable care.
The subsequent stage, which is normally carried out on a rotating, flat-disc, metallic (steel, copper or tin) lap, using fairly coarse abrasives, serves to give the rough stone the required shape and dimensions, but leaves surfaces which are translucent due to lack of polish. As with diamond cutting, this is known as bruting. The final stage, which is performed on a horizontal lap with very fine abrasives, serves to polish the individual facets. Many different abrasives are used, the commonest being emery, garnet, chromium oxide, and iron oxide.
Cutting is an operation that requires time (especially for diamonds), precision, and relatively simple tools. For these reasons, less valuable stones and small diamonds are only worth cutting nowadays where labor is cheap and in plentiful supply. Important centers for diamond cutting are: Antwerp, Amsterdam, New York, Tel Aviv, Bombay, and Cape Town, while other stones are normally cut in the countries where they are extracted, e.g. Sri Lanka, India, Thailand, Brazil, but also in Israel (Tel Aviv) and Europe (ldar-Oberstein). Automated faceting machines have recently been introduced but at present they have a relatively limited application.
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