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Sawing of diamonds

Posted September 24th, 2008 by admin

The next operation is sawing, which divides up the rough material to the best advantage. Sawing may be necessary to obtain the largest possible stone from the given piece of rough if this is flawed, for cleavage might not obviate the flawed portion. This process is carried out by means of a thin copper or copper bronze disc, about four to five inches in diameter, the thin edge of which is kept supplied with diamond dust mixed with oil. This is applied with a small roller as a paste. The rough stone is set in cement and held between two holders, while the copper disc revolves vertically at a speed of about 1,500 to 5,000 revolutions a minute.
The large face obtained by sawing is used as the table of the stone, and further work is carried out on this basis. Lately, the process of sawing has been speeded up by the use of very thin blades which are rotated at high speeds. But thin discs of a phosphor bronze or copper are still used. The old fashioned method of bruting, by which two diamonds were rubbed together to obtain a rough shape or rough facets, and to remove corners and edges of crystals, is now seldom used. Special bruting machines are in use, in which the cutting agent remains another diamond.

Grinding, by which the facets are evolved, requires a light and delicate touch as well as good eyesight. Flaws may develop into serious cracks with undue pressure, while a slight variation in the degree of hardness within the same stone, a not infrequent occurrence, calls for extra care. In many flat, triangular stones, a vein may be found which runs across the polishing grain. This vein is termed a “naat,” and such specimens require very careful cutting. The process of cutting reveals faults; what seems to be a perfectly white stone in the rough may develop a slight tinge of color, a factor which has a great bearing on the value of the stone. Many crystals, when first found, have a very thin, transparent coating over their surfaces. This coat tends to neutralize the color in the body of the stone, which cutting and subsequent polishing reveals.