e
glass (common window glass). Lead oxide being added to the mixture a
dense, very brilliant, but soft glass (flint glass) results. Cut glass
dishes and "paste" gems are made of this flint glass. Now the glasses,
although they are silicates, are not crystalline, but rather they are
_amorphous_, that is, without any definite structure. Nature's
silicates, on the other hand, are usually crystallized or at least
crystalline in structure. (In a few cases we find true glasses, volcanic
glass, or obsidian, for example.)

Having thus introduced the silicates we may now consider which ones
among the many mineral silicates furnish us with precious or
semi-precious stones.

BERYL, EMERALD, AND AQUAMARINE. First in value among the silicates is
_beryl_, which, when grass green, we call _emerald_. The _aquamarine_
and _golden beryl_ too belong to this same species. Beryl is a silicate
of aluminum and beryllium. That is, it is a compound in which oxide of
silicon is united with the oxides of aluminum and of beryllium. There
are thus four chemical elements combined in the one substance and it is
hence more complicated in its composition than any of the gems that we
have yet considered. It is worthy of note that aluminum occurs in the
majority of precious stones, the only species so far considered that
lack it being diamond, and the quartz gems.

Perhaps the silicates that are next in importance to the jeweler, after
beryl, are those which form the _garnets_ of various types. There are
four principal varieties of garnet (although specimens of garnet
frequently show a crossing or blending of the types).

GARNETS. The types are (1) _Almandite_ garnet; (2) _Pyrope_ garnet; (3)
_Hessonite_ garnet; and (4) _Andradite_ garnet. These are all silicates,
the almandite garnets being silicates of iron and aluminum; the pyrope
garnets are silicates of magnesium and aluminum; the hessonite garnets,
silicates of calcium and aluminum, and the andradite garnets, silicates
of calcium and iron.

The so-called almandine garnets of the jeweler are frequently of the
almandite class and tend to purplish red. The pyrope garnets are, as the
name literally implies, of fire red color, as a rule, but they also may
be purplish in color. The hessonite garnets are frequently brownish red
and are sometimes called "cinnamon stones." The andradite garnets
furnish the brilliant, nearly emerald green demantoids (so often called
"_olivine_" by the trade).

Thus a