The chief stone is the ruby, considered, when large, to be of even more importance and value than the diamond. There are many other red stones in this group; sapphires, also, are a species of corundum, both the blue and the colourless varieties, as are also the aquamarine, the emerald, the amethyst, the topaz, and others, all of widely differing colour, as well as the star-shaped, or “aster” ruby, called the “ruby” cat’s-eye. All these vary more in colour than in their chemical properties.

Still another variety, greyish-black and generally associated with hæmatite iron ore, is called emery, and, when ground in different degrees of fineness, is so well known by its general use as a polishing medium as to need no description. It should, however, be mentioned that amongst the more coarsely ground emery it is no uncommon thing to find minute sapphires, taking sapphires in their broad, commercial meaning, as signifying any variety of corundum, except the red and the emery.

The surfaces of crystals of corundum are often clouded or dull, whilst its classification of lustre is vitreous. It is double refracting and has no cleavage. It is found in China, India, Burma, Ceylon, South Africa, America, and in many other places, having a wide distribution.