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-lime 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).