The general principles of the art of gem cutting have been already considered; the cutting of the diamond, however, on account of the perfect cleavage and enormous hardness of the stone, requires special methods, which must be separately considered.
As we have seen, diamonds are most frequently cut in the brilliant form, which form is comparable to that of an octahedron of which two opposite corners have been truncated. A truncated octahedron, therefore, can be transformed into a brilliant by simply adding the necessary facets; hence the form of crystal, which can be most conveniently cut as a brilliant, is the octahedron. Crystals of the form shown in (Fig. n and Fig. o) are therefore specially suited for the fashioning of brilliants. The rhombic dodecahedron and the hexakis-octahedron (Fig. c and Fig. d) are also suitable; but stones, whose form differs widely from that of the regular octahedron, for example Fig. e and Fig. f; cannot be so easily transformed into brilliants. Before such a stone is faceted it is reduced by cleavage to the octahedral form, in order to avoid the tedious process of grinding away portions, which need to be removed. The property of cleavage then is very useful to the diamond-cutter, for not only is he spared much labor in grinding, but the fragments removed by cleavage can be utilized in the fashioning of smaller gems; moreover, the property can be made use of for the purpose of removing the faulty portions of a stone or of dividing a large stone of unsuitable form into several smaller ones. The operation is, however, one which demands the greatest care; the worker should be capable not only of detecting the direction of cleavage from the outward form of the rough diamond, but also of recognizing the existence of twinning in a crystal. Any attempt either to cleave a twinned crystal or to cleave an ordinary stone in a wrong direction, will probably be attended with more or less complete fracture of the stone.
The operation of cleaving a diamond, which is entrusted to trained workmen, is performed in a manner now to be described. The stone to be cleaved is fixed to the end of a rod with some kind of cement, such as a mixture of shellac, turpentine, and the finest brick-dust, and in such a position that the direction of cleavage is parallel to the length of the rod. A second diamond with a projecting edge is fixed to a similar rod with the edge uppermost. By grinding the sharp edge of the second diamond against the first, a nick in the direction in which the stone is to be cleaved is cut to a sufficient depth. The rod supporting this diamond is set on a firm elastic base, a sharp, strong chisel is placed in the proper direction in the nick, and the cleavage effected by dealing the chisel a single sharp blow with a hammer. Heating may loosen the cement, and the stone placed in another position if it is desired to cleave it in another direction. The powder produced when the nick is made is caught in a small box provided with a sieve, and is utilized in the process of grinding.
It has been stated by Tavernier that the custom of cleaving diamonds has been practiced in India since very early times; in Europe, however, the art was acquired much more recently, and is said to have originated with the English chemist and physicist Wollaston (1766-1828), of whom it is related that by cleaving away the faulty exterior portions of a large diamond he was enabled to dispose of the stone at a considerable profit.
Stones which, either by nature or by the hand of man, have been given the shape of an octahedron, have already the ground-form of the brilliant and can be at once faceted. The work of grinding the facets is facilitated by a preliminary operation, which is entrusted to special workers and is not performed in the process of cutting any other gem. This operation, by which the shape and position of the facets are roughly marked out, is known as bruting, rubbing, or graying the stone. The stone to be bruted is fixed to a handle, and, with the exception of the area on which the facet is to be made, is embedded in cement or in a fusible alloy of lead and tin. This projecting portion is rubbed with a strong pressure upon the projecting portion of another stone similarly mounted and prepared, and thus a facet, in approximately the correct position and with a fairly even but rough surface, is developed upon each of the stones. The powder abraded from the two stones during the operation is carefully preserved for use in grinding. During the operation of graying, which, by the way, derives its name from the gray metallic appearance of the facets so made, any over-heating of the stone by friction must be carefully avoided, since it leads to the development of icy flakes in the interior of the diamond. The operation is attended by a peculiar grating sound, which is said to be so characteristic that by this alone the possessor of a practiced ear can determine whether the two stones, which are rubbed together, are diamonds or some less hard gems.
At the completion of the first stage of the operation the stone is removed by warming the cement or alloy, placed in another position, and the remaining larger facets successively marked out in the same way. The smaller facets are not so marked out by a preliminary operation, but by the subsequent process of grinding. When ready for the grinding process the stones are bounded by a number of fairly even, rough faces, with a gray, somewhat metallic luster; they have no longer the appearance of diamonds, but resemble dull gray metallic bodies with the general contours of the form of cutting the stone is finally to take.
In the combined process of grinding and polishing, the preliminary disposition of the facets, which may be slightly incorrect, is rendered strictly accurate by the completion of the grinding, their rough surfaces are rendered smooth and shining, and the smaller facets are added. The grinding process is the same as in the case of other gems, the diamond being imbedded in the fusible alloy of a dop and placed on the grinding disc. Since in the grinding of diamonds the disc must be charged with diamond powder, which of course has the same hardness as the stone itself, the operations of grinding and polishing take place simultaneously, and any separate polishing process is superfluous. Any dirt or foreign matter, which may adhere to the stone after the process of grinding, is removed by treatment with fine bone-ash or tripolite.
In the process of grinding it is by no means immaterial in which direction the grinding disc moves across the facet, which is being worked. Owing to the fact that the diamond, as well as other precious stones, has a different degree of hardness in different directions, the grinding of its facets can be accomplished with comparative ease in some directions, while in others the process is extremely tedious. To avoid injury both to the stone and to the grinding disc, the diamond must be ground "with the grain"; and the operator ought to make himself familiar with these directions of least resistance, otherwise his work will be unnecessarily prolonged. When, for example, the table of a brilliant is to be developed upon an octahedron, the grinding disc should move from center to center of two opposite octahedral faces; if allowed to move from edge to edge, the facet can only be developed with the greatest difficulty, for in this direction the hardness of the diamond is much greater than in the other.
Arrows indicate the directions of least resistance to grinding on each of the facets of a brilliant. The large four-sided facets above and below and to the right and left of the table are the faces of the octahedron.
Diamonds: Large and Famous Properties Geology and Mining Diamond Cutting Diamond trade
Rafal Swiecki, geological engineer email contact
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