  Gemstone crystals often have naturally brilliant, reflecting faces, but rarely are they perfect and unblemished. Also, their natural shapes do not provide the best expression of their luster, brilliance, dispersion, color, and other inherent properties. In fashioning a gemstone, the skilled artisan tries to develop these hidden assets and to otherwise enhance the gemstone’s general beauty. From ancient times until the 1600’s little was attempted in the way of shaping gemstones other than to smooth or polish the natural form. Although similarly smoothed, or tumbled, gemstones recently have returned to fashion, the finest pieces of gem rough are now converted mainly into faceted, or shaped, stones. Standard types of facets—the flat faces that are ground and polished on the rough gem material—have been given individual and group names. A typical example is the brilliant cut, which is most commonly used to best bring out the qualities of a diamond. The standard brilliant cut, with a pattern of many facets, is commonly used for gemstones having a high refractive index and, therefore, great brilliance. Characteristic of the standard brilliant cut are the 32 crown facets surrounding a relatively small, flat, table facet and the 24 pavilion facets and culet at the bottom of the stone. Ideal proportions for the standard brilliant cut have been carefully determined so that the maximum amount of light will be reflected back out the top of the stone. Incorrect proportions cause the light to be lost at the bottom of the stone. The step cut, often called the emerald cut, frequently is used for colored stones because the large table permits a good view of the color. The emerald or step cut provides a large table and a full bottom for the stone. Although the number of crown and pavilion facets may vary, the general pattern is maintained. The simplified English brilliant cut takes maximum advantage of the strong dispersion of diamond, with its flashes of fire, but the fewer facets provide less sparkle than the standard brilliant cut. The diagram shows a brilliant-cut diamond with angles and facets arranged to give the stone maximum internal reflection as well as to make use of its strong dispersive ability. Certain of the light beams passing into a brilliant-cut diamond produce colorless brilliance by being reflected back out of the stone The English brilliant cut has 28 crown and pavilion facets—28 fewer than the standard brilliant cut. The Dutch rose cut is a very simple one that is used mainly for small diamonds in jewelry that features a larger, colored stone. It is based on a form that originated in India and was introduced through Venice. For other purposes and for other kinds of precious stones a number of basic cuts have been developed. The brilliant and step cuts are by far the commonest of these basic cuts, but modern jewelry design frequently uses such fancy cuts as the baguette, cut-corner triangle, epaulet, half moon, hexagon, keystone, kite, lozenge, marquise, pentagon, square, trapeze, and triangle. Some of these are shown here. Just as the English brilliant cut, because of its 28 fewer facets, has less sparkle than the standard brilliant cut, the step brilliant, with its 20 additional facets, has greater sparkle. The step brilliant cut is a complicated modification of the standard brilliant. With an additional 12 facets in the crown and 8 in the pavilion, the step brilliant has 78 facets, compared with the 58 of the standard. Various kinds of cuts have been devised for special purposes in jewelry design. These include the pentagon (1), lozenge (2), hexagon (3), cut-corner triangle (4), kite (5), keystone (6), epaulet (7), baguette (8), trapeze (9) and square (10). With this typical trim saw, water is used as a coolant for the rapidly rotating metal disk, which has a diamond-impregnated rim. Here, the blade is cutting its way through a piece of gem tourmaline. In general, there are three operations in preparing a gemstone from the rough—sawing, grinding, and polishing. Sawing usually is accomplished by using a thin, diamond-impregnated, rapidly rotating disk of soft iron or bronze, with oil or water being used as a coolant. The very hard diamond dust literally scratches its way through the stone. Once the stone is sawed to shape, the facets are ground and polished on a rotating horizontal disk by the use of various abrasives. For rough grinding, The final step in preparing a gemstone from rough is the applying of a high polish by pressing the stone against a rotating disk that has an extremely fine abrasive on its surface. Here, the disk is of felt, and the abrasive is tin oxide. The cabochon cut gets its name from the French word “caboche,” meaning pate or knob, a reference to the rounded top of the stone. Here, from top to bottom, beginning at left, are cabochons of turquoise, agate, and petrified wood; jasper, smithsonite, and williamsite; and amazonite, petoskey stone, and carnelian. (Two-thirds actual size.) These exquisite bowls, measuring 2 to 3 inches across, are part of a set of 35 carved by George Ashley of Pala, Calif., from gem materials found in the United States. Left to right: paisley agate from California, petrified wood from Arizona, black jade from Wyoming, chrysocolla from Arizona, and variscite from Utah. (One-third actual size.) Shaping of gemstones is not limited to geometric faceting. Many stones, especially those which are opaque or which produce stars and cat’s-eyes, are cut as cabochons. This ancient, and probably oldest, cutting style consists merely of a raised and rounded form. When extended completely around the stone, the cabochon form results in a bead that can be drilled and strung. Many cabochons, especially those of less expensive gem materials, are now cut in large quantities to standard sizes in order to fit mass-produced gem mountings. Sculpting in gemstones is a much more intricate, nongeometric kind of shaping. Although tools differ in detail, and the gem sculptor must possess an artistic eye as well as lapidary skill, the basic processes of sawing, grinding, and polishing are the same. This coral carving, 11 inches tall without the stand, owes its thin, graceful, willowy shape to the skill of the artist in following the contour of a natural coral branch. The contemporary sculptor Oskar III J. W. Hansen visualized and created the likeness of a spirited stallion in this 4½-inch turquoise carving, a gift of George Gilmer. This world-famed crystal ball, given to the Collection as a memorial to W. R. Warner by his widow, represents another phase of the lapidary art. Cut from a block of Burmese quartz estimated to weigh 1000 pounds, this extremely valuable, flawless, colorless sphere has a diameter of 12? inches and weighs 106¾ pounds. |
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