Simon & Schuster's Guide to Gems and Precious Stones. Questo articolo non è disponibile.Simon & Schuster's Guide to Gems and Precious Stones

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.

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 thes...