Crystallized Alumina Minerals: Corundum

CORUNDUM:

Modern mineralogists have given the single name corundum to all the minerals consisting of crystallized alumina almost pure, without regard to the colour of these minerals.

Corundum comprehends three varieties: the hyaline corundum, the laminated corundum, and the granular corundum. The first variety comprehends the precious stones.

The primitive form of the crystals of corundum is the six-sided prism (Fig. 52), but the most general form of the hyaline corundum is the dodecahedron (Fig. 53), with faces formed of isosceles triangles.

Nearly all the hyaline corundums susceptible of being employed in jewelry are brought from Pegu. The price of these stones is very high. It is not a rare occurrence for the price of the ruby to exceed that of the diamond. At an auction of precious stones from the cabinet of M. Dree a very beautiful diamond of 8 grains (2 carats) was sold for $148, while a ruby of the same weight brought $186. At the same sale the price of a ruby of 10 grains reached $2604. The value of the ruby and that of precious stones generally varies with their richness of tone.

When the corundum is perfectly colourless it possesses a brilliancy so vivid that in some circumstances it may pass for a diamond. It may be distinguished from the latter by its inferior hardness, its smaller specific gravity (3.5, while that of the diamond is 3.9), and by having double refraction.

According to the tints which the corundum possesses, it bears different names, and represents variable values.

Names of the corundums according to their colour:--

White Sapphire, Colourless.

Oriental Ruby, Crimson red.

Oriental Ruby (variety), Rose red.

Oriental Sapphire, Azure blue.

Indigo Sapphire, Indigo blue.

Oriental Amethyst, Violet.

Oriental Topaz, Yellow.