PRECIOUS STONES. 43
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with finding what is the highest member of the series of test stones it
will scratch ; in some cases we may be able to try to scratch the cut
gem on the girdle or part by which its mount clasps it. Many minerals
show a different degree of hardness on different crystal faces or in
different directions ; thus Cyanite shows a variation of hardness in
different directions between 5 and 7 on Mobs' scale. We must be careful
to distinguish between a scratch on a test stone and a streak of
broken-down fragments of the tested specimen ; if it be a true scratch
the mark will of course remain on brushing with a soft camel-hair
brush. It is necessary to examine the mark with a lens in cases of
doubt. In testing, no more force should be used than is just
sufficient to produce the scratch, and no more scratch should be made
than is necessary. "Where the test stones are of a cleavable mineral,
clean cleavage planes (c.f. Cleavage) will be found the most
suitable for trying the hardness on. A steel point is as good an
instrument as one can have for a single test; good carbon steel
tempered to a pale straw colour will just scratch quartz under
favourable conditions; more often the steel can be scratched by quartz.
Thus a steel point may be taken as of No. 7 hardness; what is called "
gem-hardness," that of the true gem stones, is greater than 7. It
should be noted that the newer " high speed " steel is a great deal
harder than ordinary carbon steel. As glass is easily scratched by
steel, such a steel point will readily serve to distinguish glass
imitations of precious stones in most cases.
A
tabular statement of the hardness of precious stones is given below. It
should be noted that under certain species are included in some cases
several varieties, and
