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CUTTING AND POLISHING 181
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posed
of brickdust and resin, he applies the edge of a steel knife to the
scratched surface, and strikes a quick, hard blow with a slender rod.
If he has struck the lines of cleavage, the external scale is at once
removed, for the diamond, despite its hardness, is quite easily
fractured. Then the split surface must be polished. If no other scale
or marked inequality needs removing by splitting, the next operation is
that of grinding.
Cutting
As the diamond is the hardest of all known substances, it is evident that much patience and strength are required in reducing
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its
size or altering its rough figure by grinding. The ordinary file would
serve to reduce some other gems, but it will not touch the diamond.
Diamond cut diamond is not merely a current phrase, for diamond dust is
now invariably used in polishing or grinding this precious stone.
For
cutting and polishing purposes the lapidary has a table above which a
flat steel wheel revolves horizontally. On the upper surface of this
wheel are fine grooves or striae, cut angling
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