Gemstones.

Gemstones. I INTRODUCTION Gemstones, minerals that are treasured for their beauty and durability. A large number of minerals have been used as gems. Their value generally depends on four elements: the beauty of the stone itself; its rarity; its hardness and toughness; and the skill with which it has been cut and polished. Stones such as diamonds, rubies, and emeralds represent one of the greatest concentrations of money value. During times of war or economic disturbance many people convert their wealth into precious stones, which are transportable and more easily sold. II OPTICAL PROPERTIES The beauty of gems depends to a large extent on their optical properties. The most important optical properties are the degree of refraction (see Optics) and color. Other properties include fire, the display of prismatic colors; dichroism, the ability of some gemstones to present two different colors when viewed in different directions; and transparency. Diamond is highly prized because of its fire and brilliancy, ruby and emerald because of the intensity and beauty of their colors, and star sapphire and star ruby because of the star effect, known as asterism, as well as for their color. In certain gemstones, notably opals, brilliant areas of color can be seen within the stone; these areas change in hue and size as the stone is moved. This phenomenon, known as play of color, differs from fire and is caused by interference and reflection of the light by tiny irregularities and cracks inside the stone. Opals also exhibit milky or smoky reflections from within the gem. Gems that are fibrous in structure show irregular interior reflections similar to those seen on watered or moiré silk. This optical property, which is called chatoyancy, is exhibited by several gems, notably the tigereye and cat's-eye. The appearance of a gem as seen by reflected light is another optical property of gemstones and is called luster. The luster of gems is characterized by the terms metallic, adamantine (like the luster of the diamond), vitreous (like the luster of glass), resinous, greasy, silky, pearly, or dull. Luster is particularly important in the identification of gemstones in their uncut state. III IDENTIFICATION OF GEMS A gem cannot always be identified by sight alone. It is therefore necessary to rely on measurement of the optical properties that can be determined without harming the stone in any way. The gemologist uses an instrument called a refractometer to measure the characteristic property of the stone, known as refractive index, which is its relative ability to refract light. In addition, an instrument called the polariscope is employed to determine whether a gem is doubly or singly refracting (see Crystal). Emeralds, rubies, sapphires, amethysts, and synthetic rubies and sapphires are all doubly refracting, whereas diamonds, spinels, synthetic spinels, garnets, and glass are singly refracting. A special dark-field illuminator with a binocular microscope is employed for examining the interior of a gemstone to determine whether it is of natural or artificial origin, and to search for inclusions characteristic of a given gemstone. These tests usually are sufficient to identify the rather limited number of materials used as gemstones; occasionally, however, other instruments are required, including a dichroscope, which measures the property called dichroism, or a spectroscope to determine the characteristic absorption spectra (see Spectroscopy; Spectrum). Hardness, the test ordinarily associated with gem testing, is never used on cut stones by the gemologist. Another physical test that can be given to an unknown stone is the determination of its specific gravity (see Density). For exact determinations various weighing devices are used, but rough approximations of the specific gravity of lighter stones can be made by means of a series of liquids of known specific gravity. If the stone will float in a liquid having a specific gravity of 4 and sink in a liquid with a specific gravity of 3, the specific gravity of the stone must lie between these limits and be approximately 3.5. IV GEM MATERIALS The accompanying table lists precious and semiprecious gem minerals with the names commonly applied to them. See separate articles on many of the gems mentioned. V ARTIFICIAL GEMS The term artificial gem is used to describe either an imitation of a natural gemstone or a synthetic gem that is chemically identical to naturally occurring gems. A Imitation Gem Such a gem may be made of flint glass, often silvered on the back to increase the brilliance. Since World War II, colored plastics have replaced glass, especially in costume jewelry. Plastics are cheaper, more easily molded, and lighter in weight. During the 19th century, artificial pearls were made by blowing hollow beads of glass and pouring into them a mixture of liquid ammonia and the white matter from the scales of fish such as the bleak, roach, or dace. A much better type of artificial pearl, the indestructible bead, was introduced shortly after 1900. The bead is made of solid glass with only a narrow hole for the thread. Pearl essence, consisting of the crushed scales of certain herring, is applied to the outside of the glass and covered with a transparent, colorless lacquer. The most successful imitation of a diamond is strontium titanate, made by a flame-fusion technique. Its index of refraction is almost identical to that of a diamond, and it has a higher dispersion. Thus, it has the brilliance and greater fire than the diamond. It scratches easily, however. A harder material simulating the diamond is rutile, or titanium oxide. B Synthetic Gems The term is limited by the U.S. Federal Trade Commission to manufactured materials that duplicate a natural gemstone chemically, physically, and optically. Synthetic gems can be distinguished under a microscope because they are more perfect than natural gemstones and contain no irregularities. Synthetic diamonds were first made by the General Electric Company in the U.S. in 1955. In their process, carbonaceous compounds are subjected to pressures of 56 metric tons per sq cm (360 metric tons per sq in) at temperatures of 2760° C (5000° F). The diamonds thus produced are suitable only for industrial use. In the late 1960s a method was developed for "growing" diamonds by heating a diamond particle to a high temperature and subjecting it to methane gas. The gas decomposes into carbon atoms, which adhere to the diamond crystal. The crystal structure of the enlarged diamond is identical to that of a natural diamond. Diamonds of about 1 carat (200 mg or 0.007 oz) have been produced by this method, but their cost is still considerably higher than that of naturally occurring diamonds. Sapphires are made in an apparatus resembling an oxyhydrogen torch. The flame is directed into a fireclay support inside an insulated chamber. The oxygen gas carries finely powdered pure aluminum oxide into the flame, and the powder fuses into droplets, forming a cylindrical boule, or matrix, on the support. The size of the resulting sapphire is controlled by varying the gas flow, temperature, and amount of powder. Boules weighing up to 200 carats (40 g or 1.41 oz) can be produced by this technique. Perfect rubies and sapphires up to 50 carats (10 g or 0.353 oz) have been cut from such a boule. Rubies are made by the same process by adding 5 to 6 percent chromium oxide to the aluminum oxide. Colors other than red are produced by adding different metallic oxides. Stars can be added to synthetic rubies or sapphires by adding an excess of titanium oxide to the aluminum oxide powder and heating to temperatures greater than 1000° C (greater than 1832° F). In gems made with this technique, synthetic stars appear sharper than naturally occurring stars. Emeralds, some of which are of gem quality, are synthesized by still-secret methods. They can be distinguished from natural emeralds by their red glow under ultraviolet light. VI GEM CUTTING The shaping and polishing of gem materials to enhance their beauty and, in some cases, to remove imperfections is performed by expert workers known as lapidaries. Their trade, although highly skilled, is not as exacting as that of the diamond cutter. A Materials and Equipment Gems are shaped entirely by being ground on abrasive wheels or revolving abrasive disks. For minerals that are no harder than quartz, natural sandstone wheels are sometimes used, but for the harder stones, such as rubies and sapphires, synthetic grinding wheels of cemented Carborundum (silicon carbide) must be employed. The first step in the cutting of a gem is to saw it roughly to shape. Thin abrasive disks or metal disks charged with powdered diamond or other abrasives are employed in this process. Wheels (called laps) made of Carborundum or of abrasive-charged cast iron are used to shape the stone. The stone to be shaped is cemented to the end of a wooden stick called a dop and is held against the revolving wheel or lap with the aid of a supporting block placed adjacent to the wheel. This supporting block contains a number of holes in which the end of the dop can be rested. By changing the dop from one hole to another the lapidary is able to control the angle of the facet, or face, being ground. When the stone has been ground to the required shape, it is brought to a high polish on wooden or cloth wheels charged with a fine abrasive such as rouge or tripoli powder. B Gem Cuts The oldest and simplest of the many standardized shapes or cuts given to gemstones is the cabochon cut, in which the stone is smoothly rounded. The cabochon cut is essential if a star or cat's-eye is to be visible, and is the most satisfactory cut for opal, moonstone, and colorful opaque gems. Cabochon-cut stones usually are rounded on the back; this is sometimes advantageous in improving appearance, but often is done in order to give the stone extra weight. Various forms of faceted cuts, in which the gem is given a number of symmetrical plane surfaces, or facets, are universally employed in the cutting of diamonds and are used extensively for other stones as well. The most common cut is the brilliant. In this cut the top of the stone is ground to a flat so-called table from which the sides of the stone slope outward to the broadest portion of the stone, which is known as the girdle. Below the girdle, the sides slope inward at a slightly broader angle to a tiny flat surface, the culet, parallel to the table at the bottom of the stone. The ordinary brilliant-cut stone has 32 facets besides the table in the top portion of the stone (called the crown or bezel) above the girdle, and 24 facets besides the culet on the bottom portion of the stone (called the pavilion or base) below the girdle. In rare cases the number of facets is increased by some multiple of 8. Scientific studies have worked out proportions of the size and inclination of the facets that give the maximum brilliance to a given gem. In addition to the round brilliant, stones are cut in a variety of square, triangular, diamond-shaped, and trapezoidal faceted cuts. The use of such cuts is largely determined by the original shape of the stone. Large rubies, sapphires, and emeralds are often cut square or rectangular with a large table facet surrounded by a relatively small number of supplementary facets. The emerald cut, which is frequently also used for diamonds, resembles the brilliant, but has a large square or rectangular facet at the top and a total of 58 facets in all, although more or less facets may be used, again added or subtracted in multiples of 8. See also Jewelry. VII GEM ENGRAVING Designs are cut in precious or semiprecious stones either as cameos, in which the design is raised in relief above the surface, or as intaglios, in which the design is incised into the surface (see Cameo). Intaglios were formerly often used as seals for making impressions on wax or damp clay. The technique of gem engraving requires, on all hard stones, the use of a rotating metal tool. The stone is fastened to a wooden handle and moved against the tool, which does not itself perform the cutting of the design but merely rubs abrasive powder on the stone. The ancients probably used emery powder for this purpose, but since Roman times the abrasive has been a mixture of diamond dust with oil. A Ancient Engraving Intaglio cutting probably started during the 4th millennium BC, in Mesopotamia, during the Elamite and Sumerian civilizations (see Mesopotamian Art and Architecture). The first seals, made of stone, were usually cylindrical and were suspended on a cord. The art reached its peak about 2800 crystal; these commonly dealt with the adventures of the mythical king Gilgamesh. By the 1st millennium BC BC, in elaborate cuttings on cylindrical rock the art had spread throughout Asia Minor and Egypt. Although the cylindrical form was still common, domed and conical seals with flat surfaces for the intaglios became popular. The Egyptians initially adopted the cylinder but later produced seals of various shapes, including that of the scarab beetle, often cut in one of the colored quartzes, such as amethyst, carnelian, or jasper. Unlike the people of Asia Minor, they engraved symbols rather than pictorial scenes (see Egyptian Art and Architecture). Although the Egyptians made use of the quartzes for their engravings, the most popular material for the making of seals was glazed earthenware. The earliest Cretan gems were carved in soft steatite, but by about 1700 BC harder stones such as chalcedony were employed. The engraving of seals for the bezels of rings was first practiced about 1100 BC. The carvings on the gems of Greece and Rome provide a complete miniature history of the art of every period during which they were made (see Greek Art and Architecture). The Greek gems of the 6th century BC were cut in agate, carnelian, and chalcedony; by the 4th century BC the last had become the most popular material, although lapis lazuli, agate, jasper, and rock crystal were also employed. Gems of the Hellenistic period, dating from about 330 BC, were cut in a large variety of stones, including garnet, beryl, topaz, sard, agate, and amethyst. The use of glass as a substitute for more precious stones was introduced about this time. The cameo, usually made of one of the layered quartzes (such as sardonyx) or in colored glass, made its first appearance in Hellenic Greece and was brought to a high artistic level by the Roman craftsmen. The cameo was commonly employed in articles of personal adornment, such as brooches or clasps. The intaglio gems of Rome were usually used as the bezels of rings. B Revival in Europe By the 2nd century AD gem engraving had declined in Asia Minor, the best examples being talismans produced by the adherents of the Gnostic heresy. They are frequently connected with the symbolism attached to the worship of Mithras. In Europe a limited number of gems were engraved, usually for bishops' rings, until the 7th century, but the art then declined until the end of the 14th century, when Florentine and German engravings made their appearance. In Italy the art received impetus from the ardor with which the Medici family collected gems. Although the artists of the Renaissance based their designs on those of the Greek and Roman artists, they employed a freedom of interpretation that made their work individual. On the other hand, the revival of gem engraving that took place in the 18th and 19th centuries produced works that so closely resembled the classical originals that it is difficult to tell them apart. Contributed by: Gemological Institute of America Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.