Thus all the garnets are silicates and yet we have these four principal mineral species, which, however, are more closely related to each other in crystal form, in character of composition and in general properties, than is usual among the other silicates. Specimens which have any one of the four types of composition unblended with any of the other types would be found to be exactly alike in properties. As was suggested above, however, there is a great tendency to blend and this is well illustrated by the magnificent rhodolite garnets, of rhododendron hue which were found in Macon County, North Carolina. These had a composition between almandite and pyrope, that is, they had both magnesium and iron with aluminum and silica.
The true topaz next calls for consideration as it too is a silicate. The metallic part consists of aluminum, and there are present also the non-metals fluorine and hydrogen. Here we have five elements in the one substance. Various specimens of this species may be wine yellow, light blue, or bluish green, pink or colorless, yet they all have essentially the same properties.
Tourmaline is about as complicated a mineral as we have. It is a very complex silicate, containing aluminum, magnesium, sodium (or other alkali metal, as, for example, lithium), iron, boron, and hydrogen. As Ruskin says of it in his The Ethics of the Dust, when Mary asks "and what is it made of?" "A little of everything; there's always flint (silica) and clay (alumina) and magnesia in it and the black is iron, according to its fancy; and there's boracic acid, if you know what that is: and if you don't, I cannot tell you to-day and it doesn't signify; and there's potash and soda; and on the whole, the chemistry of it is more like a mediæval doctor's prescription, than the making of a respectable mineral." The various tourmalines very closely resemble each other in their properties, the slight differences corresponding to differences in composition do not alter the general nature of the material.
Moonstone belongs to a species of mineral known as feldspar. The particular feldspar that furnishes most of the moonstone is orthoclase, a silicate of potassium and aluminum. Another feldspar sometimes seen as a semi-precious stone is Labradorite. Amazonite, also, is a feldspar. Sunstone is a feldspar which includes tiny flakes or spangles of some other mineral.
The mineral species olivine gives us peridot. It is a silicate of magnesium.
Zircon is itself a species of mineral and is a silicate of zirconium. The names hyacinth, jacinth, and jargoon are applied to red, yellow, and colorless zircon in the order as given.
Jade may be of any of several different species of minerals, all of which are very tough. The principal jades belong, however, to one or the other of two species, jadeite and nephrite. Jadeite is a sodium aluminum silicate and nephrite, a calcium magnesium silicate.
Leaving the silicates we find very few gem minerals remaining. The phosphates furnish us turquoise, a hydrous aluminum phosphate, with copper and iron. Variscite is also a phosphate (a hydrated aluminum phosphate).
The carbonates give us malachite and azurite, both carbonates of copper with combined water, the malachite having more water.