ich would be too soft to
scratch it, yet the stone would scratch the felspar, but not zircon or
andalusite, 7-1/2, or topaz, 8, so that his tests would at once classify
the stone as a piece of cut and coloured quartz, thus confirming what he
would, at the first sight, have suspected it to be.

The standard stones themselves are much more certain in results than the
needles, which latter, though well selected and tempered, are not
altogether reliable, especially in the more delicate distinctions of
picking out the hardest of certain stones of the same kind, in which
cases only the expert judge can decide with exactness. Accurate in this
the expert always is, for he judges by the sound and depth of his cut,
and by the amount and quality of the powder, often calling the
microscope to his aid, so that when the decision is made finally, there
is never the least doubt about it.

Rapidly as these tests can be made, they are extremely reliable, and
should the stone be of great value, it is 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