is composed mainly of oxide of silicon, but the opal also
has water combined with the silicon oxide (not merely imprisoned in it).
Thus opal is a hydrous form of silica (hydrous comes from the Greek word
for water).

SPINEL. All our other stones are of more complicated chemical
composition than the preceding. Coming now to mineral species which have
three chemical elements in them we may consider first _spinel_, which
has the two metallic elements aluminum and magnesium and the
non-metallic element oxygen in it. It is virtually a compound of the two
oxides, aluminum oxide and magnesium oxide. The variously colored
spinels, like the various corundums, all have the same properties, thus
they are all of hardness 8 or a little higher, they all have single
refraction, and all have specific gravity 3.60.

CHRYSOBERYL. Another mineral species which, like spinel, has just three
elements in its composition is _chrysoberyl_. This mineral contains the
metals aluminum and beryllium combined with the non-metal oxygen. Thus
it is really to be regarded as a compound of the two oxides, aluminum
oxide and beryllium oxide. This species furnishes us _Alexandrite_,
_chrysoberyl cat's-eye_ and less valuable chrysoberyls of
yellowish-green color. All are of the one species, the marked color
difference being due to the presence of different impurities. The
cat's-eye effect in one of the varieties is due to the internal
structure rather than to the nature of the material.

THE SILICATES. Nearly all of the remaining precious stones belong to a
great group of mineral species known as the silicates. These are so
called because they consist largely of oxide of silicon (the material
above referred to under quartz gems). This oxide of silicon is not free
and separate in the silicates but is combined chemically with other
oxides, chiefly with metallic oxides. Thus there are many different
silicates because, in the earth, many different metallic oxides have
combined with silicon oxide. Also in many cases two or three or even
more metallic oxides have combined with silicon oxide to make single new
compounds.

GLASS, A MIXTURE OF SILICATES. Those who are familiar with glass making
may receive some help at this point by remembering that the various
glasses are silicates, for they are made by melting sand (which is
nearly pure oxide of silicon) with various metallic oxides. With lime
(calcium oxide) and soda (which yields sodium oxide) we get soda-lim