ility, thousands of fine opals, and a
host of commoner ones, are set in rings, where many of them
subsequently come to a violent end, and all, sooner or later, become
dulled and require repolishing.

The great beauty of the opal, rivaling any mineral in its color-play,
causes us to chance the risk of damage in order to mount it where its
vivid hues may be advantageously viewed by the wearer as well as by
others.

VERY SOFT STONES. Of stones softer than 6 we have but few and none of
them is really fit for hard service. Lapis lazuli, 5-1/2 in hardness,
has a beautiful blue color, frequently flecked with white or with bits
of fool's gold. Its surface soon becomes dulled by hard wear.

Two more of the softer materials, malachite and azurite, remain to be
described. These are both varieties of copper carbonate with combined
water, the azurite having less water. Both take a good polish, but fail
to retain it in use, being only of hardness 3-1/2 to 4.

LESSON XVIII

MINERAL SPECIES TO WHICH THE VARIOUS GEMS BELONG AND THE CHEMICAL
COMPOSITION THEREOF

Although we have a very large number of different kinds of precious and
semi-precious stones, to judge by the long list of names to be found in
books on gems, yet all these stones can be rather simply classified on
the basis of their chemical composition, into one or another of a
comparatively small number of mineral species. While jewelers seldom
make use of a knowledge of the chemistry of the precious stones in
identifying them, nevertheless such a knowledge is useful, both by way
of information, and because it leads to a better and clearer
understanding of the many similarities among stones whose color might
lead one to regard them as dissimilar.

MINERAL SPECIES. We must first consider what is meant by a "mineral
species" and find out what relation exists between that subject and
chemical composition. Now by a "mineral species" is understood a single
substance, having (except for mechanically admixed impurities)
practically a constant chemical composition, and having practically
identical physical properties in all specimens of it.

DIAMOND AND CORUNDUM. A chemist would call a true mineral a _pure
substance_, just as sugar and salt are pure substances to the chemist.
Thus _diamond_ is a "mineral species," as is also _corundum_. There are
many different colors of both diamond and corundum, but these different
colors are believed to be due to the presence in t