Diamonds have the highest refraction index, and those used for jewels are very transparent. Diamonds from Indian deposits were known in ancient times; in the West the limited use of diamonds began in the late Middle Ages. Diamonds for jewelry are graded on the basis of colour from blue-white to yellow. Grading also is done on the basis of purity, which varies from perfectly clear, extremely pure stones to those with many impurities and flaws. Large demand provided an incentive for the production of false diamonds (as well as other stones) as early as 1675 in Paris.
Mogok rubies, from Myanmar (Burma), are the most highly prized because of their bright red colour (pigeon blood). Those from Thailand are usually a more brownish colour, while those from Sri Lanka tend toward violet. Production of synthetic stones is far greater than the supply of natural rubies. The physical and optical properties of synthetic and natural rubies are so similar that it is difficult to distinguish between them.
The sapphire (blue variety of corundum) is considered one of the most valuable of precious stones. A sapphire's colouring usually indicates its origin. Those from Myanmar are deep blue. The Kashmir (Indian) sapphire is cornflower blue and is highly prized, being quite rare. Sapphires from Thailand are very similar in colour to those from Myanmar; those from Sri Lanka are of different shades but incline toward violet. Sapphires, like rubies, can be cut so that, in the light, a beautiful, luminous six-pointed star appears on the surface of the gem. Star sapphires and rubies are semi-opalescent. Synthetic sapphires and rubies are produced by the same industries.
The green emerald is a precious stone used since very ancient times. There is documentation of its presence in Egypt during the life of Pharaoh Sesostris III in the 19th century BC. At the end of the 16th century, emeralds from South America were brought into Europe. On the American continent, the first peoples to use emeralds were those belonging to the pre-Columbian civilizations, in particular the Inca. In 1935 in the United States (Chatham) and in Germany (Farbenindustrie), synthetic emerald crystals were made with characteristics similar to natural ones.
Among the beryls, mention must be made of morganite (pink beryl) found in various shades of peach-blossom pink. The main deposits are in California and Madagascar.
The two best known and most widely used varieties of chrysoberyl are alexandrite (transparent) and Oriental cat's-eye (opaque). Because of its great power of absorption of certain colours, alexandrite looks green in daylight and reddish purple in artificial light. The cat's-eye is a yellowish green colour and is characterized by a luminous line. The intensity of the light in this line varies according to the brightness of the rays of light that strike it.
One of the most important gems with pure crystals is the topaz, used a great deal in jewelry. The honey-yellow variety is the best known, but there are also pink, red, blue, and the less-used colourless stones. The Oriental topaz (a corundum) and citrine quartz are also widely used. They are less rare than other kinds of topaz and, therefore, less expensive but create a similar effect.
Among the less-important and less-rare stones, the zircon is quite widely used in its three varieties: orange, blue, and colourless. The orange variety is called jacinth and was used to a great extent in classical antiquity. The blue variety is called starlite or Siam zircon, while the third type is called Ceylon or Matara diamond.
Among the semiprecious stones used in jewelry are amethyst, garnet, aquamarine, amber, jade, turquoise, opal, lapis lazuli, and malachite. Matrix jewelry is cut from a stone such as opal or turquoise and the surrounding natural material, or matrix.
The pearl is one of the oldest gems known. Its colour varies according to the waters from which it comes. Pearls from the Persian Gulf are usually cream-coloured; those from Australia are white with greenish or bluish shades; golden-brown pearls come from the Gulf of Panama; those from Mexico are black or reddish brown; pink pearls are from Sri Lanka; and those from Japan are cream-coloured or white with greenish tones. The main characteristic of the pearl is its iridescence. Baroque pearls are those with defects in their outer layer. In modern times baroque pearls are rounded off artificially but, in the 16th and 17th centuries, their irregular form was exploited in jewelry by using them to make up parts of animals or other figures. After huge quantities of cultivated pearls invaded the world market, interest in natural pearls underwent a considerable decrease.
In addition to pearl, a number of other organic materials, including amber, coral, ivory, and jet, are considered gems.
Amber is a fossil resin, usually yellowish brown, but on occasion deep brown to red, green, or blue. It is an amorphous hydrocarbon and may contain particles of various foreign materials, trapped insects, and air bubbles. Its lustre is greasy to resinous. The most noted occurrence of amber is along the shores of the Baltic Sea, where pieces have been washed up by wave action. Other important occurrences are along the coast of Sicily, in Romania, and in Myanmar near Myitkyinā.
Coral is the skeletal material of calcium carbonate built up by small animals that live in colonies in the sea. This material is usually branchlike and occurs in a variety of colours, of which the most sought after are rose red to red. The best coral comes from the Mediterranean Sea, particularly off the coasts of Algeria and Tunisia. A black horny coral growth, probably conchiolin, which hardens on exposure to air, has been obtained off the islands of Hawaii. Coral is carved into art objects and cut as beads, cameos, and other ornaments.
The use of ivory for ornamental purposes dates to prehistory. The term should be restricted to the material derived from the tusks of certain animals—namely, the elephant, hippopotamus, warthog, walrus, sperm whale, narwhal, and the extinct mammoth (fossil ivory). The pale cream colour of new ivory darkens with age to yellow. All types are brittle and will not peel as do the plastics used to simulate them.
Jet is a dense variety of lignite formed by the submersion of driftwood in the mud of the seafloor. It has been recovered since Roman times from the shales near Whitby in northeastern England. It takes a high polish and was once popular as mourning and ecclesiastical jewelry but has been superseded by black onyx, black tourmaline, and plastics. Because it is actually a variety of coal, it will burn.
Gem engraving, setting, and cutting
The most ancient technique of stone engraving, intaglio-incised carving, was probably first used to produce seals. The art is believed to have originated in southern Mesopotamia and was highly developed by the 4th millennium BC. During the Hellenistic Age (c. 323–30 BC) intaglio surface engraving gave rise to the idea of carving stones in relief, exploiting the different coloured layers of certain minerals to create contrasting figures (cameo): the background was cut down to the lower level, of a different colour or shade, in order to make the subject stand out chromatically. The stones that have properties suited for this purpose are sardonyx, agate, and onyx.
The cameo is usually one of the components for necklaces, bracelets, and rings or is included in medallions with a jeweled frame. The art of cameo in jewelry was most highly developed during three periods: the late republican to early imperial period in Rome, the Renaissance, and the Neoclassical period in the 18th century.
The evolution of techniques of setting has followed that of stonecutting. The insertion of gems in jewelry can be done in various ways. The setting can have a round, square, oval, or rectangular collet (rim); in periods in which gems were mounted in their own irregular shapes, the collet followed this form. Usually, on the inside of the collet a short distance from the edge, there is a protrusion on which the stone rests. The edge is pounded down around the gem to ensure its stability. In coronet settings the form may be conical or pyramidal, solid or pierced. The edge is first shaped into a row of teeth, which are later hammered down onto the gem in order to hold it in place. Until fairly recently, nearly all gems were mounted on a metal base; and transparent stones, according to their colour, were placed on a gold or silver base to increase the amount of light reflected. As new cuts were developed for stones, setting techniques also progressed, especially for those jewels in which important stones like diamonds, emeralds, and rubies form the main theme. The tendency was to leave the stones as visible as possible (especially in rivière necklaces and bracelets made only of diamonds) by mounting them with a very small ring of white gold or platinum fitted closely against the back of the stone. Three claws, attached to this ring, hold the stone in place.
Pearls, like some coloured stones, in ancient classical times were pierced with a drill, the hole going half or all the way through according to whether the pearls were to be strung on a necklace or fastened onto a jewel.
Until the 15th century, stones were only polished or the part to be left visible was rounded into a dome shape called cabochon. The cutting known as faceting gradually developed from the first attempts in the 15th century, probably in France and the Netherlands. During the 16th century the simple rose cut began to be used, after which there were no new developments until 1640, when, under the patronage of Jules Cardinal Mazarin, the first brilliant cut was carried out (also called the Mazarin cut). Toward the end of that century, a Venitian succeeded in obtaining the triple brilliant cut, which is still used. The numerous cuts used for diamonds today are usually applied to other precious and semiprecious transparent stones as well. For emeralds, rubies, and other coloured stones the square or rectangular cut with a stepped bulb or the cabochon cut are usually used.
