also
subjected to other unerring' tests of extreme severity, any one of
which would prove it false, if it chanced to be so, though some stones
are manufactured and coloured so cleverly that to all but the expert
judge and experienced dealer, they would pass well for the genuine.
In
Mohs's list it will be seen that several stones vary considerably, the
opal, for instance, having a degree of hardness from 5-1/2 to 6-1/2
inclusive. All stones differ slightly, though almost all may be said to
fit their position in the scale : but in the case of the opal, the
difference shown is partly due to the many varieties of the stone, as
described in the last chapter.
In
applying this test of hardness to a cut gem, it will be noticed that
some parts of the same stone seem to scratch more readily than others,
such as on a facet at the side, which is often softer than those
nearest the widest part of the stone, where the claws, which hold it in
its setting, usually come. This portion is called the "girdle," and it.
is on these " girdle " facets that the scratches are generally made.
This variation in hardness is mostly caused by cleavage, these cleavage
planes showing a marked, though often but slight, difference in the
scratch, which difference is felt rather than seen. In addition to the peculiar feel of a cutting scratch, is the sound of
it. On a soft stone being cut by a hard one, little or no sound is
heard, but there will form a plentiful supply of powder, which, on
being brushed off, reveals a more or less deep incision. But as the
stones approach one another in hardness, there will be little powder
and a considerable increase in the noise ; for the harder are the
stones, cutting and being cut, the louder will be the
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