This many-colored mineral, composed of nearly pure alumina, produces
gems which in some cases are more valuable even than diamonds. The
ruby, sapphire, Oriental emerald, Oriental topaz, Oriental amethyst,
Oriental aquamarine, Oriental chrysolite, Oriental hyacinth, star ruby,
star sapphire, star topaz, and ruby and sapphire cat’s-eyes are all
corundums of different colors. The ruby is a red sapphire, and the
Oriental topaz a yellow sapphire, while the Oriental emerald is a green
sapphire, etc., etc.
In hardness corundum ranks next to the diamond, ranking No. 9 in Moh’s
scale.
The specific gravity is 3.9 to 4.1, the crystallization rhombohedral,
and cleavage basal, the crystals breaking across the prism with nearly
a flat surface.
In lustre, the corundum is vitreous, its refraction double but not to
a high degree, and it is susceptible of electricity by friction, which
the polished specimens especially retain for a considerable time.
Corundum is unaffected by chemicals, and is infusible alone, but in
combination with a flux it melts with difficulty into a clear glass.
The chemical composition of precious corundum is:
Alumina 98.5
Oxide of iron 1.0
Lime 0.5
─────
100.
Thus it will be seen that corundum is composed almost wholly of
alumina,—one of the constituents of common clay, which, when colored
by traces of metallic oxides, chrome, etc., produces a greater variety
of precious stones of a high rank than any other mineral.