Text 19. Gem cutting

Gemstones are 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.

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.

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.

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.

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NOTES:

• lapidary – гранильщик драгоценных камней;

• facet – грань, фаска, фацет;

• grinding – шлифовка, измельчение, истирание;

• abrasive - абразив;

• girdle – тонкий пласт песчаника;