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The process of cutting gems is simple, but the results are due to the skill and especially to the judgment of the cutter. That part of the surface of a rough stone at which it is desired to place a facet is rubbed with a harder stone or with some other effective substance. The harder stone or substance abrades small fragments and powder from the softer, and gradually the surface of the subject mineral is transformed into a plane face, or facet. In like manner other facets are added or a rounded surface is produced by similar means. In grinding, the harder stone or abrasive material is reduced to a fine powder and mixed with olive oil into a paste (if diamond powder), or with water (if emery), and placed near the edge of a circular disk, or "lap," which is about twelve inches in diameter and an inch in thickness. The lap, usually of metal, revolves horizontally with great velocity, and the precious stone to be ground is pressed against the disk where the disk is loaded with the abrasive paste; the pressure causes the powder to become embedded in the soft metal of the disk. This acts as a file, equal in hardness to the grinding powder. The duration of the operation depends upon the hardness of the precious stone and of the abrasive matter. The skill required of the operator involves the most careful watchfulness against exceeding the size prescribed in the plan for the stone; also against overheating the stone, which causes the development of small cracks in the interior of the stone called "icy flakes." An essential prerequisite for grinding precious stones is a means by which they can be held steadily and true in a desired position. For this the diamond-polisher uses a time-honoured tool called a "dop" (commonly pronounced "dub"). This holder of the rough diamond is a small hemispherical cup of iron attached by the convex side to a stout copper rod. The cup is filled with an easily fusible alloy of tin and lead, which is fused and allowed to cool; just before this composition solidifies the stone to be cut is set in the position desired in the cooling alloy, with about half its bulk projecting from the metal. Thus the stone is firmly fixed in an immovable position. The semi-precious stones, when cut by the lapidary, are set in the end of a wooden holder, or "stick," with some kind of resinous cement.
Diamond cutters formerly cut the diamonds in a small wooden box especially designed for this use; all of the operator's strength was needed to rub two diamonds together, a process called "bruting," so that the attrition under this pressure would cut the stone into the shape desired. About the year 1888 the first machine was invented to shape diamonds, and the cutter, who formerly had to cut the stone twice, or several times, accomplishes the same result in one operation. All diamond-cutting in America is now done by machine, while in Europe the smaller sizes are still cut by hand in the old tedious and laborious method. The tools for polishing remained unimproved from the inception of the modern diamond-cutting industry until the year 1896, when the machine dop or holder was invented. This modern machine dop, although still an imperfect device, holds the stone without the application of the mixture of lead and tin, but it can only be used for stones of a fair size. The majority of the cutters and polishers of diamonds in the United States now use these mechanical dops, as the market and industry in America demands stones of considerable size almost entirely; it is impossible to use these dops for the stones of small size exclusively cut in Europe. The inventor of the machine dop also invented the machine for sawing diamonds. Through the use of this device pieces of the stones which were formerly polished away and ground to worthless black dust are now saved. The economy effected by the sawing machine is illustrated by its use in cutting off the apexes of the rough diamond crystals; the smaller parts, called melee, are sent back to Europe to be cut. |
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Precious Stones Guide Vol 4
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