. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. Fig. 45. Directions of least hard ness on the facets of a brilliant. unnecessarily prolonged. When, for example, the table of a brilliant is to be developed upon an octahedron, the grinding disc should move from centre to centre of two opposite octahedral faces; if allowed to move from edge to edge, the facet can only be developed with the greatest
. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. Fig. 45. Directions of least hard ness on the facets of a brilliant. unnecessarily prolonged. When, for example, the table of a brilliant is to be developed upon an octahedron, the grinding disc should move from centre to centre 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. The directions of least resistance to grinding on each of the facets of a brilliant are indicated in Fig. 45 by arrows. The large four-sided facets above and below and to the right and left of the table, are the faces of the octahedron. We have thus seen that the three operations in the process of diamond-cutting are entrusted to as many classes of skilled workmen, namely cleavers, bruters, and grinders or polishers. The order in which the facets of a brilliant are ground has also a certain importance. Starting from the octahedral ground-form, the table and culet are in every case first developed. The correct proportions of a brilliant are attained by grinding away five-ninths of the upper half for the table, and one-ninth of the lower half of the stone for the culet, the upper portion of a perfect brilliant being one-third, and the lower being two-thirds of the whole thickness of the stone from table to culet. Some rough stones are of such a shape that they cannot be cut into the usual brilliant form, but are given an oval or triangular outline; in this case, the method of procedure described above requires a slight modification, as also when the stone is to be cut as a rose or in some form other than the brilliant. In the latter cases the cleavage of the stone does not play so im
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