The Garnets possess many curious forms of crystals, which are coloured and used as gems. Tourmaline is a very particularly useful crystal, and is used in the investigations concerning the polarization of light. It is found of nearly all colours. The garnet and staurolite crystals are shown (figs. 470, 471).
The former is silicate of alumina with the silicate of some other oxide, which is not always the same. This change, of course, gives us a series, as in the case of alum above mentioned.
Fig. 470.—Garnet crystal.
The red varieties, called almandine, or precious garnets, are distinguished from the duller, “common” species by their clear colour. Bohemia is the most productive soil for the garnets.
Mica includes, as we have already noticed, a group of minerals which have a peculiarly laminated structure. These layers are by no means all alike, but they present a smoothness to the fingers which is highly characteristic. The chief constituents are alumina and silica, occasionally with magnesia. Mica slate is very common, and is often used instead of glass in window-frames. Muscovite, lepidolite, and phlogopite are all micas of the “potash,” “lithia,” and “magnesia” varieties.
In the list of minerals associated with the lighter metals, we need only now mention the Gems, so well known. These stones are very hard in many instances, infusible, and exhibiting beautiful colours. Amongst them are diamonds, sapphires, and rubies, of which we have spoken; the topaz, noticed under corundum. The chrysoberyl (of a pale green, or occasionally reddish hue), of which the alexandrite of Siberia is a variety, is a compound of glucina with alumina; the beryl, a silicate of the same, and the emerald of beautiful green. Zircon is another gem, and “hyacinth” is its most valued form. The latter is found in basaltic rocks. The emerald crystallizes in the hexagonal system.
Fig. 471.—Staurolite crystal.
We may now consider the minerals formed by the heavier metals, such as Iron, Copper, Nickel, etc.