Gem Cutting

gem cutting

[′jem ‚kəd·iŋ]
(lapidary)
The polishing of rough gem materials into faceted or rounded forms for use in jewelry.

Gem Cutting

 

the processes (cutting, roughing, faceting, and polishing) of treating the surface of natural and synthetic semiprecious minerals and glass for jewelry and for industrial use.

The initial raw material for making jewelry stones (mountings, beads, and pendants) includes natural and synthetic semiprecious stones with a high index of refraction of light (above 1.54), as well as translucent and opaque colored stones with attractive coloration and natural design, which are used for small articles, and clear colorless or colored glass, without internal flaws, which simulates faceted semiprecious stones.

The faceting of jewelry stones consists in giving them the geometrically straight or asymmetrical form of a polyhedron with facets placed symmetrically on its surface. The best form (to preserve the mass of the mineral), as well as the type of cut that permits the maximum optical effect of the “play of light” in the stone, is selected for each raw material. For transparent stones the most widespread types of cut are rose, with 12 to 72 lateral facets; simple semibrilliant, with 12 to 32 lateral facets; and brilliant, with 48 to 240 or more lateral facets. Brilliant cutting gives the stone maximum luster and play of light.

The classical type of brilliant cut contains 56 lateral facets. Stones of square, rectangular, rhomboid, and other shapes are cut in steps, wedges, or a mixed fashion. The cabochon cut is used for opaque or semitransparent minerals, and sometimes also for transparent minerals (turquoise, lapis lazuli, opal, moonstone, sapphire, and emerald), if they have natural flaws. The distinguishing feature of most minerals that are cut is their great hardness, which necessitates the use of a diamond instrument for almost all stages of the cutting process.

The preparation of brilliants begins with the sawing of the diamond into two or more parts on a machine with a fine (up to 0.1 mm) copper wheel with diamond powder. The split diamonds (for round brilliants) are ground to form on a special machine, on which the instrument is again a diamond. The cut diamond is faceted on a cast-iron disk (face plate) coated with diamond powder, using a quadrant and numerous devices for cutting the facets at strictly determined angles. The finished brilliants are classified by weight, flaws, and color and are evaluated accordingly. Large pieces of semiprecious minerals are cut into semifinished products, just as diamonds are, by diamond circles 0.3 mm and more thick. The preliminary form is made on a grinding or cutting machine by means of a diamond face plate. Facets are made on a cutting machine with an attachment for reading the facets (a quadrant goniometer). Depending on their physicochemical properties, transparent minerals refract and reflect at various angles a beam of light passed through them; for this reason the facets of gem minerals are placed at strictly determined angles.

Semiprecious gems are polished by face plates (disks) made of alloys of lead and tin, whose surfaces are notched with a sharp steel plate. The notches, which are made by incisions, prevent the polishing powder from being thrown off by the rotation of the disk. The polishing powder is selected depending upon the hardness of the mineral being worked; it may be chromium oxide, iron oxide, aluminum oxide, or tripoli. In the jewelry industry the simplest types of cuts of jewelry stones of synthetic corundum are produced on semiautomatic machines; up to 80 rough products can be made simultaneously in this way. In the USSR, jewelry stones of synthetic corundum are worked by a domestically produced high-output diamond instrument. Corundum styli for sound recording are cut in the same manner as jewelry stones of synthetic corundum.

The working of semiprecious stones to make jewelry was known as early as the third millennium B.C. (ancient Egypt and Mesopotamia). The invention of the cutting machine in Holland in 1456 played an exceptional role in the development of gem cutting. This made it possible to introduce the most advanced type of cut, the brilliant. In the 19th and 20th centuries, gem cutting has become mechanized with the appearance of new, more advanced machines, devices, and instruments.

Russian gem cutting reached its golden age in the 18th century. The construction of a cutting factory in Petergof (near St. Petersburg) in 1725, a cutting factory in Ekaterinburg (in the Urals) in 1774, and a grinding factory in Kolyvan’ (in the Altai) in 1786 dates to this period. Russian gem cutting had its greatest success toward the beginning of the 19th century; in addition to factory workers (especially in the Urals), a large number of master craftsmen, often entire families, were cutting gems.

After the Great October Revolution gem cutting changed from a handicraft and semihandicraft branch to a mechanized branch of the economy. As a result of the exploitation of diamond deposits in Yakutia (1954), the cutting of diamonds in the USSR acquired much broader scope (in the Moscow, Sverdlovsk, and Smolensk factories). Since 1965, domestic brilliants and jewelry made from brilliants and cut semiprecious stones have been exported.

There is lot production of new types of cutting equipment and instruments in the USSR, Great Britain, the United States, and Switzerland. The main centers of gem cutting (especially diamond cutting) abroad are the Netherlands, Switzerland, Israel, and the United States. The cutting of semiprecious stones is well developed in the Federal Republic of Germany, and glass cutting is advanced in the Czechoslovak Socialist Republic.

REFERENCE

Andreev, V. N. Ogranka samotsvetov, vols. 1–2. Moscow, 1957–58.

A. S. PERKINA

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