Chapter 1

1 DIAMOND

Native carbon. The same element also occurs naturally in the form of graphite, another mineral with completely different characteristics and appearance.

Its name comes from the Greekadámas,meaning "invincible," in recognition of its exceptional hardness, which makes it resist any form of abrasion by other minerals.

Crystal systemCubic.

AppearanceDiamonds most commonly occur as isolated crystals, which may be in the form of a more or less perfect octahedron, an octahedron with curved faces, or sometimes an icositetrahedron or hexoctahedron, which are more complex forms somewhat similar to an octahedron. The crystal can also be in the form of a rhombic dodecahedron or a tetrahexahedron with rounded corners and slightly curved faces, to the point of being almost spherical. Certain flattened, basically triangular twinned forms are also frequent. More or less cubic forms are rare. Rough-looking surfaces characteristically display superficial irregularities either in the form of fairly large cavities or hundreds of smaller irregularities, only recognizable under a lens, the extreme hardness of diamond generally ruling out the signs of abrasion seen on rough surfaces of other minerals that are found in secondary deposits. Pieces of diamond are often found that are clearly cleavages of other larger stones. Less typical, but quite frequent, are forms consisting of agglomerations of crystals, with concentric zoning and numerous impurities. Generally of irregular or globular appearance, with a rough or almost smooth surface, they are calledbort (or boart).Another microcrystalline form occurring as irregular aggregates of roughly octahedral, cubic or rhombic dodecahedral appearance, is calledcarbonado,on account of its blackish color. Bort and carbonado are used for industrial purposes only. Diamond's microcrystalline structure compensates for its brittleness due to easy cleavage. Crystals with flat faces can be transparent, with strong luster, but blackish carbon inclusions, cloudy patches or fractures are often visible on the inside.

When the faces are curved or fairly rough, the crystals are generally merely translucent, even though it may be evident from cleavage surfaces that these imperfections are in an outer "skin," and that the crystals are transparent on the inside. Transparent stones are usually more or less colorless, but can be various shades of yellow-to-dull-yellow or more rarely, yellow with a brownish tinge. But bright yellow and clear brown are possible; and, as an extreme rarity, there are diamonds that are blue, pale green, pink, violet, and even reddish. The translucent stones with a skin often look grayish white (like ground glass); or dull yellow, yellow-brown, pale green, or pink. But they are often different on the inside: fairly clear, tinged with yellow or, more rarely, brown. The strongest colors are usually confined to the less transparent, outer layer. The bolt varieties can often be yellowish, yellow-brown or grayish, while carbonado is blackish.

Physical propertiesDiamond is rated 10 on Mohs' scale of hardness. It is the only mineral with this degree of hardness, although such a property is difficult to quantify. Depending on the methods of measurement, it is estimated to be from 10 to 150 times harder than corundum, the only mineral with a hardness of 9. Because all the remaining minerals have a hardness of less than 9, clearly there is a vast difference between them and diamond. But diamond has fairly easy cleavage parallel to the octahedral faces, which can make it brittle. The density is 3.52 g/cm3. The refractive index ofn2.417 is well in excess of the measuring capabilities of the average refractometer. Singly refractive, diamond crystals can display areas of anomalous birefringence. It has fairly high dispersion, equal to 0.044, which is the highest for colorless minerals (the effect of dispersion is not appreciated in colored stones, so it is not considered).

GenesisThere is still considerable uncertainty as to the origin of diamond. The most widely accepted theory is that it was formed at great depths in the earth's crust, at very high pressures and temperatures. Explosive types of volcanic phenomena would then have been responsible for driving it to the surface, with such a rapid drop in temperature that it was impossible for the diamond to be transformed into graphite, which is the carbon phase stable at Iow pressures. It would presumably have been carried to the surface in breccia of the peridotitic type known as kimberlite, which constitutes the infill of diamond-bearing pipes (structures with the appearance of explosive volcanic vents).

Its outstanding resistance to physical and chemical erosive agents means that crystals are found in a variety of environments, in secondary deposits where they have arrived unchanged after two or more cycles of erosion and sedimentation, making it impossible to establish a relationship between present deposits and places of origin.

OccurrenceFor many centuries, the only place where diamonds were found was India, where, however, very small quantities were mined. Early in the eighteenth century, diamonds also began to be mined in Brazil, which shortly afterwards became the principal world supplier. In the second half of the nineteenth century, they began to be mined from deposits in South Africa, which in turn, soon became the chief world source. Since the beginning of the twentieth century, diamonds have also been found in Angola and Zaire (responsible for up to 60 percent of annual world production, mainly for industrial uses), Ghana, Guinea, Ivory Coast, Tanzania (which has one of the largest primary deposits in the world), and the Soviet Union (which is currently the second largest producer in the world). Diamonds are also found in Guyana, Venezuela, and, in very limited quantities, Borneo. They have recently begun to be mined in China (in the province of Hunan), and considerable quantities have been discovered in Australia, where extraction has already begun. Bear in mind, however, that diamonds are only said to be worth exploiting where they occur in average concentrations of one part in twenty million, or in other words, where twenty tons of rock have to be worked for each gram of diamonds.

1.1 Diamond

Ancient civilizations were fascinated by the exceptional hardness of diamond, although colored gems were regarded as more aesthetically pleasing. Diamond was extremely rare up to the eighteenth century and was only fully appreciated after the modern type of brilliant cut, which shows it in all its glory, was developed at the beginning of the twentieth century. It is the most important gemstone today. Statistics a few years ago showed that diamonds accounted for eighty percent of the movement of money generated by gemstones. About two million carats of cut diamonds are issued on the market each year (it is the only gemstone for which reliable statistics are available), equal to a volume of little more than 110 liters.

AppearanceIn most cases it is almost colorless or, to be more precise, ranges from perfectly colorless (infrequent) to yellow-tinged or, sometimes, brownish. Diamonds with a definite color are extremely rare. This can be yellow, yellow-brown, or predominantly brown or, very occasionally pink to reddish, blue, blue-gray, pale green, or violet. Its luster, depending on reflection from both the inner and outer surfaces of the light incident on the table and crown, is greater than that of other gemstones, due both to its high refractive index, which facilitates total internal reflection and its exceptional hardness, enabling it to acquire a similar degree of polish.

By far the most widely used cut is the round, brilliant type, which best displays the gem's unique characteristics. But oval, marquise, pear and, more rarely, heart-shaped fancy cuts are also used. Most of these have a girdle consisting of a series of small, polished facets, while in brilliants, a girdle cut this way is uncommon and is reserved for stones treated with particular care. The special, rather elongated forms often show a dull area along the minor axis. Obviously, the better the cut, the less this band will show. The so-called emerald cut is also quite common. This has a rectangular table, stepped and chamfered. Unfortunately, this cut, which is used to reduce wastage when the stone is fashioned, is more often than not given the wrong proportions. The crown is usually too shallow (even less than 10 percent of the smaller side of the girdle) and the pavilion too deep (50-55 percent of the smaller side). The result is a stone with a lot less fire than one with a brilliant cut, or even than the rare examples of gems with correctly proportioned emerald cuts.

Diamonds are also found on the market with unusual, antique or specially designed cuts. Old mine cuts are not normally circular, but squarish, with rounded corners, or almost rectangular with rounded corners (some people call these polygonal shapes with slightly curved sides and smoothed corners "cushion" shape). The proportions of the height of the crown, the pavilion and the diameter vary a great deal in these cases, depending on the creativity of the cutter. It is still possible to find what are known as "rose" cuts, with a flat base, both in stones of some size, which are usually old or antique, and in small, shallow stones one or two millimeters in diameter, generally used in old-fashioned jewelry.

Distinctive featuresHardness can be an important factor in distinguishing diamond from other stones. It is in fact the only gemstone capable of scratching corundum. The best modern imitation, cubic zirconia, is less hard than corundum, so the two can easily be distinguished by comparing them with corundum, although the results of the tests must be observed under a binocular microscope or at least a lens. Diamond's exceptional hardness is also displayed by the facet edges, which are sharper than in imitations. This is best appreciated in relation to zircon, which has brittle, easily damaged edges, and the less hard imitations such as synthetic ruffle and strontium titanate. If the stones are turned between the thumb and forefinger, the two softer substitutes feel more slippery, almost oily, compared with diamond, because of this difference in the edges.

Another characteristic can be seen in the girdle, but only when it is not faceted. Due to the procedure used in turning brilliant cuts, the girdle of a diamond will have a satiny finish, similar to that of finely ground glass (in recent years a similar effect has been achieved with cubic zirconia, but no other imitation displays this). Also, when there are numerous flaws like minute cracks extending from either side of the girdle (these are known as "bearding" and are due to inexpert turning of the rough stone), it is bound to be diamond. A brilliant cut can display small facets on the girdle or extending from the girdle toward the pavilion (or more rarely, toward the crown). These are the remains of the outer surface of the uncut octahedron (nearly always with minute, crystallographically oriented shallow triangular cavities), or of the faces of octahedral cleavage. In the latter case, small steps can be observed between contiguous, specular plane surfaces. However, these details are only readily visible if magnified at least 10-20 times. Sometimes small triangular facets alone are seen extending from the girdle. Called extra facets or supplementary facets, these are produced by polishing of the facets just described or by the elimination of some small, almost superficial flaw. Although very similar to the foregoing and not usually found on imitations, these facets are less distinctive in that they could be produced on any other stone.

Other distinctive features are related to the fact that the most frequently used, brilliant and emerald cuts, are designed to make the most of the high refractive index of diamond and obtain the maximum possible total internal reflection of the light coming from the table facet. Therefore if a diamond is placed with the table facet against tiny written characters, nothing will be visible through it, unless the pavilion is extremely flat. With imitation diamonds of a much lower refractive Index, such as YAG (Yttrium Aluminum Garnet), something will be visible through the stone and still more will be seen through synthetic spinel and colorless sapphire. The difference is more obvious with emerald cuts than with brilliants. A similar effect, but confined to brilliant-cut stones, can be seen through the table facet. By steadily tilting a stone of lower refractive index than diamond, and looking through the table, a nonreflecting transparent triangle can be seen to appear in the pavilion, with its apex at the center and its base toward the edge of the table opposite the observer. The lower the refractive index of the stone, the smaller the angle at which this will appear. In diamonds, this effect is very difficult to see, except in poorly cut stones with very shallow pavilions.

Single refraction is another characteristic that distinguishes diamond from zircon, which is strongly birefringent and from an infrequent imitation of diamond: synthetic rutile, which is even more strongly birefringent. The famous dispersion in diamond, although considerable, is much less than that of synthetic ruffle and strontium titanate; but these now uncommon imitations look positively iridescent when viewed through a lens and even to some extent with the naked eye.

Given the constancy of shape and proportions, at least within certain limits, of stones with a round brilliant cut, a given weight can be said to correspond to a given diameter. If the diameter of a stone presumed to be a diamond can be measured with some precision, one can check to see whether it has a suitable weight (in which case it will either be diamond or a stone of comparable specific gravity), or whether the weight immediately rules out the possibility of its being diamond. Generally simulants are too heavy, as in the case of cubic zirconia, GGG (Gadolinium Gallium Garnet) or, to a much less obvious extent, zircon or YAG (Fig. A.). This method could not be used to distinguish diamond either from synthetic spinel, colorless topaz, or colorless sapphire, as their relative densities are too similar, although all of them have other characteristics unlikely to deceive any but the most casual observer.

It was mentioned in discussing physical properties, that the very high refractive index of diamond is outside the range of normal refractometers and the same can be said of many of its imitations. In compensation, however, diamond has other characteristics, such as reflectivity and thermal conductivity, which are quite different (because much higher) than those of its present substitutes. Small instruments the size of a pocket calculator have been produced to measure these characteristics, making a rapid distinction possible.

OccurrenceGem quality diamonds are found in about twenty different countries, a dozen of which are in Africa, three in Asia, one in Oceania, and three in South America. By far the largest producer is South Africa, including the neighboring Lesotho, Botswana, and Namibia. Next come the Soviet Union, Angola, Zaire, and Sierra Leone. Other important areas are the Central African Republic, Tanzania, Ghana, and Venezuela, with Australia and Brazil further behind, and India now one of the last.

ValueDiamond is one of the most valuable stones, together with ruby and emerald. The market value of diamonds is determined by complex grading systems that divide them into a wide range of different categories. The basic characteristics considered are weight, color, and purity. Less crucial factors are the proportions of the cut and surface finish.

The influence of weightAlthough a diamond weighing over 3000 carats has been found, rough diamonds of considerable weight are extremely rare. Still rarer are heavy cut diamonds, given that the average loss of weight in the cutting process is more than 60 percent. All else being equal, a diamond twice the weight of another is much rarer and therefore much more precious and will have a higher price per carat. Weight also has psychological as well as scarcity value: a diamond of I carat or slightly more is worth more weight for weight than one of 0.9 carats because it exceeds the conventional limit of the complete carat. The same applies at the dividing line for 2 carats, 3 carats, and so on (Fig. B). In short, the price per carat increases by stages with each complete carat number, up at least to a weight of about 10 carats, after which the effect is negligible, although such large diamonds are extremely uncommon.

The influence of colorThe vast majority of diamonds are colorless, or yellowish, ranging from a barely perceptible tinge to straw-yellow. Perfectly clear stones, however, are comparatively rare, but the effect of dispersion in diamonds is more pleasing on a clear ground than on a yellowish one. For these, plus psychological reasons, the more or less perfectly colorless stones are in greater demand; and value diminishes with an increase in yellow (or more rarely, brown) coloration.

In an effort to quantify this, scales of colors of different intensities were established, with names of the color categories relating to the principal localities in which diamonds of that color were found, or to other factors. Thus the main diamond trading centers evolved roughly similar color codes matched by approximate sets of values. A very precise scale has more recently been introduced in the United States, with grades distinguished by letters of the alphabet (beginning with D, not A) and referring to data supplied by special measuring instruments rather than the naked eye. Over the last few years, European scales have been adapted to that of the United States, a series of sample diamonds being used for purposes of comparison. The four scales most widely used today and in the recent past are shown in Fig. C (4b is the same as 4a, but with the old nomenclature of 1). The correspondence between scales 1 and 2 is approximate, as is their relationship to the others, whereas 4a and 4b were designed to be cross-referenced. As a general guide, color H (white) is very good and few diamonds 'can boast such quality. Color I (the old "commercial white") is much better than this name would imply, with a barely perceptible yellow tinge; and many stones on the market, particularly medium- or large-sized ones, come under categories J, K, L, or M. By contrast, "rare white" stones are much less frequent. A very rough idea of the visibility of coloration in diamonds of different grades (seen from above only, in conditions of "use," rather than the ideal conditions according to which they are classified) can be obtained from Fig. D, which uses the classifications of the CIBJO scale. Naturally, the larger the area over which any coloration is viewed, the easier it is to see. All else being equal, the price of diamonds varies quite sharply with color. Again, as a rough guide and obviously depending on the state of the market, if a diamond with certain characteristics and weight of color H were worth 100, another with the same characteristics of color F could be worth 130, while one of color J would be worth 80, and one of color M, 40.

The influence of purityMany diamonds contain crystalline inclusions or discontinuities (even just fractures) which reduce their transparency, by interrupting the paths of the light rays. Stones free of inclusions or visible discontinuities are described as pure, and value diminishes as visible imperfections increase.

Classification is based on visual criteria and detailed diagrams are available to establish a basis of comparison. Stones are conventionally examined under a 10x lens. The grades of the scale of purity are shown in Fig. E and are referred to by internationally accepted sets of initials, according to the English terminology. Pure stones of IF grade are uncommon; VVS and VS grades are of a good level of purity, SI and PI grades are not so pure and P2 and P3 are the lowest categories. Naturally, stones combining perfect purity with excellent color are rare, especially those weighing several carats. If one is prepared to compromise over a single characteristic such as color, it will be easier to find stones of good purity and weight. Similarly, it will be easier to find a stone of good color and weight but mediocre purity; and those of middling color, medium purity, and small size are obviously the most common.

The above should give some idea of the complexity and also the degree of organization of criteria used for the valuation of diamonds. In addition to these fundamental criteria there are others based on the proportions of the cut (when not ideal, they reduce the stone's powers of light reflection and refraction) and surface finish, which are taken into account in establishing value. Proportion and finish are judged as "very good," "good," "medium," or "poor." Sometimes these judgments are based on measurement, but usually they are established by the visual evaluation of experts.

Simulants and syntheticsDiamond has been imitated by glass and special glass with a mirror backing to increase reflection, called "strass." It has been imitated by colorless, synthetic spinel and corundum ever since these began to be produced. Over the last few decades, moreover, it has been imitated by a whole range of synthetic and artificial products:

* strontium titanate or fabulite, which has much higher dispersion than diamond, with striking iridescence, low hardness, considerable brittleness and density;

* synthetic ruffle or titania, which has very high dispersion with striking iridescence, marked birefringence, high density, and low hardness;

* YAG (Yttrium Aluminium Garnet), which has a slightly lower refractive index than diamond, a hardness of less than 9, and greater density;

* GGG (Gadolinium Gallium Garnet), which has a fairly low hardness and double the density of diamond;

* cubic zirconia (cubic zirconium oxide), djevelite or phianite, the most recent and hardest to distinguish from diamond. Its density, however, is much greater, and its hardness is less than 9.

Many other artificial substances are similar to diamond, but are not used as imitations because their cost is generally much higher and their characteristics no better than those simulants already mentioned. Diamond has been and still is manufactured synthetically, in minute crystals, only suitable for industrial purposes. It has also been produced experimentally for use as a gemstone and a few gems of about 0.25-0.45 carats have been cut. But it is so hard to make the crystals grow that the cost would be much higher than that of natural diamonds obtained from known deposits.

2 CORUNDUM

Al2O3

Aluminium oxide. The name is probably derived from an old Indian word,corund,which referred to an unknown mineral or gem.

Crystal systemTrigonal.

AppearanceIt occurs in semiopaque masses similar to whitish or grayish vein quartz, but also in distinct, prismatic or tapered crystals, with close transverse striations, some of which resemble elongated bipyramids. Often opaque or translucent, corundum can be partially or perfectly transparent. All the colors of the spectrum are possible, from red to yellow, green, blue, and violet; in addition, the stones may be pink, gray, black, or colorless and all the shades between. Brightly colored, transparent, translucent, or semiopaque varieties make highly aesthetic and valuable gems. Because of its hardness and resistance to chemical attack, corundum is often found in alluvial deposits in the form of pebbles that retain clear indications of their original crystal shapes.

Physical propertiesCorundum has a hardness of 9, the highest in the mineral world after diamond. The density is approximately 4.0 g/cm3. The refractive indices are about n??? 1.760, n??? 1.769. Parting parallel to the basal plane is sometimes visible, with an appearance of cleavage.

GenesisIt is formed by contact metamorphism between alumina-rich magmas (and related pegmatites) and limestone, or by regional metamorphism of alumina-rich, silica-poor rocks.

OccurrenceThe least attractive variety of corundum, known as emery (usually a corundum-magnetite mixture) and used as an abrasive, is mainly found in Greece, the United States, and Australia. The gem varieties come chiefly from Sri Lanka, Thailand, Cambodia, Burma, and Australia, with smaller deposits in India, Tanzania, and the United States.

2.1 Ruby

The most valuable variety of corundum is ruby. The name comes from the Latinrubrum,"red." Like other red stones, it has also been calledcarbunculus,or carbuncle, meaning a small coal or ember.

AppearanceThe color varies from fiery vermilion to violet red, but because rubies are pleochroic, different colors are also found in the same stone; bright or sometimes brick -ed in one direction, tending to carmine in the other. The color is also accompanied by marked fluorescence which ,s stimulated by ordinary, artificial light and above all, by the ultraviolet rays of direct sunlight. Thus rubies turn brighter red under such light and the purplish ones look "redder." If the color is too pale, they are no longer called rubies, but pink sapphires; if it is more violet than red, they are known as violet sapphires. But it is hard to establish precise limits, as all the intermediate shades are possible. The brightest red and thus the most valuable rubies (usually from Burma often have areas full of inclusions in the form of minute ruffle needles (or straws), which interfere with the light, producing a distinctive silky sheen known, in fact, as silk. When the silk is not heavy, the stones are clearer, more attractive, and even more valuable. Other, mainly crystalline inclusions are normally found as well. Rubies of this type are not usually more than a few carats in weight. The rare exceptions generally contain copious inclusions. Violet red, sometimes quite dark, rubies come principally from Thailand. The type most often found on the market nowadays, they can be several carats in weight. They are normally clearer, without patches of silk. While good-sized clear stones are found, specimens with many inclusions are commonly sold as well. Rubies are usually given a mixed cut, which is generally oval, but can be round or, more rarely, other shapes. In the past, they were given a cabochon cut, like all stones outstanding for their color. Today, however, this cut is reserved for less transparent stones with numerous inclusions.

Distinctive featuresRubies can often be distinguished by their immediately visible characteristics: a fairly obvious pleochroism, a distinct brightening of color in strong light, the silk effect (where present), and a considerable luster. While spinel can be a similar color and has a similar luster, it is not pleochroic, turns much less bright in strong light, and never displays the silk effect. Red garnet is not pleochroic and the color does not brighten in strong light; it has a similar luster, but when given a faceted cut often displays dark, blackish areas within the stone. Red tourmaline is usually a completely different shade, but can be very similar, with a pleochroism comparable to that of ruby. It does not, however, brighten in strong light, and this can be sufficient indication to warrant testing its physical properties, which are quite different. The other red gemstones mentioned also differ physically from ruby. Some caution is needed with garnets, which show wide variations in both density, which in some cases coincides with that of corundum, and refractive index, which can coincide with one of the figures for corundum. Garnet; however, is singly refractive, and examples with an index in the region of 1.76 have a lower density than that of ruby.

OccurrenceThe rubies with the finest color come from the Mogok region in Burma. These are most truly vermilion, though they still have a touch of carmine. Thailand, however, is today the main source of rubies. Thai rubies are usually slightly less attractive, a bit darker with a violet tinge, but they often have fewer inclusions. Rubies are also found in Sri Lanka, but in very small quantities. Often pale, almost pink, they can be attractive, with an appearance that is both brilliant and lively. Small quantities of very fine rubies also come from the area of Cambodia on the border with Thailand, while rather opaque specimens, mainly of inferior quality, are found in India and Pakistan. Tanzania and neighboring countries have also been mining rubies for a few years. Some of the rubies found in these countries are almost as finely colored as those from Burma, with inclusions similar to rubies from Thailand, while others are semiopaque and of very limited value.

ValueThe highest quality, best colored and most transparent stones (usually from Burma), weighing, for example, 3 to 5 carats, can be as valuable as diamonds, or even more so. Very good quality rubies of even greater weight are extremely rare and fetch exceptionally high prices. Good quality stones of at least 2 carats (a bit more violet in color and usually from Thailand) are still quite valuable (particularly the more transparent ones). The price falls considerably for stones of less than a carat, which are too dark in color, and have inclusions clearly visible to the naked eye.

Simulants and syntheticsRuby has very occasionally been imitated by glass, which has a rather different, less lively color and an inferior luster. It has sometimes been imitated by doublets, with the top part consisting of garnet, to provide luster, hardness, and natural-looking inclusions and the bottom part of red glass, fused rather than cemented to the garnet layer. But such imitations are uncommon. Synthetic ruby has been produced from the beginning of the twentieth century and was the first synthetic gemstone to be manufactured on an industrial scale. To make these synthetic stones harder to distinguish from some natural rubies with numerous inclusions, they have sometimes been fractured internally by heating and rapid cooling. More recently, doublets imitating rubies have also been produced in the Orient. The top part of these doublets consists of poorly colored (usually pale green or yellow) natural corundum with obvious, typical inclusions; and the lower part is synthetic ruby, held to the corundum by transparent cement. The effect is highly deceptive: the reassuring presence of natural inclusions and characteristic luster combined with a color which is not perfect, but is normal for the majority of rubies, can be much more convincing than a synthetic ruby.

2.2 Sapphire

This is the blue variety of corundum. The name is probably derived, through the Latinsapphirusand Greeksápheiros,from a Sanskrit word. As with other gem names, however, the Latinsapphirusdid not originally denote the gem it is associated with today. Judging by the description of Pliny the Elder, it almost certainly referred to what is now known as lapis lazuli, rather than corundum.

AppearanceSapphires can be a very dark blue, to the point of seeming dense and blackish from a distance, sometimes a

Excerpted from Simon and Schuster's Guide to Gems and Precious Stones by Curzio Cippriani, Alessando Borelli
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