Simon & Schuster's Guide to Gems and Precious Stones - Softcover

Simon & Schuster

Softcover

ISBN 10: 0671604309 ISBN 13: 9780671604301

Publisher: Touchstone, 1986

Synopsis

Simon & Schuster's Guide to Gems and Precious Stones provides both the connoisseur and the casual collector with a compact, easy-to-use volume describing more than 100 rare varieties of minerals whose beauty and mystery have possessed our imaginations from time immemorial. More than 450 brilliant photographs accompany profiles of each gem, covering such aspects as appearance, physical properties -- density, hardness, refraction -- occurrence, and how to judge quality and value. Additional sections describe the process of cutting gemstones and the techniques professional gemologists use to evaluate a stone's weight and optical properties.
Detailed and comprehensive, this book is essential for anyone interested in the study of gems and precious stones.

About this title

Excerpt. © Reprinted by permission. All rights reserved.:

Chapter 1

1 DIAMOND

C

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 Greek adámas, meaning "invincible," in recognition of its exceptional hardness, which makes it resist any form of abrasion by other minerals.

Crystal system Cubic.

Appearance Diamonds 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 called bort (or boart). Another microcrystalline form occurring as irregular aggregates of roughly octahedral, cubic or rhombic dodecahedral appearance, is called carbonado, 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 properties Diamond 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 of n 2.417 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).

Genesis There 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.

Occurrence For 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.

Appearance In 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 features Hardness 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.

Occurrence Gem 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.

Value Diamond is one of the most valuable stones, together with ruby and emerald. The market value of diamonds is determined by complex grading systems t...

Language Notes:

Text: English, Italian (translation)

About this edition

• Publisher: Touchstone
• Publication date: 1986
• ISBN 10: 0671604309
• ISBN 13: 9780671604301
• Binding: Paperback
• Number of pages: 384
• Editor: Lyman Kennie