| 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.
The Ruby.
The red sapphire or ruby is the most valuable of the corundum family, and when found of a good color, pure and brilliant, and in sizes of one carat and larger, it is much more valuable than a fine diamond of the same size.
Fine rubies larger than 1½ to 2 carats are very rare, and when a fine stone from 3 to 5 carats is offered for sale, the price mounts into the thousands.
The color varies from the lightest rose tint to the deepest carmine; that color, however, which has the greatest value is known in commerce as pigeon’s blood, and is the color of arterial blood, or of the very centre of the red ray in the solar spectrum.
The imperfections in rubies, as in all corundums, consist largely of clouds, milky spots, and cracks. A perfect ruby is rarely met with, and a stone possessing brilliancy and the true color, even if slightly defective, is considered more valuable than an absolutely perfect ruby of an inferior color.
Rubies are found in Siam, Ceylon, Burmah, Brazil, Hindustan, Borneo, Sumatra, Australia, France, and Germany.
Where rubies and sapphires are met with it is said that gold is almost sure to be present.
Chemists have succeeded in producing minute crystals of rubies and sapphires which, under the microscope, presented the true crystallization of corundums, and upon being tested proved to be of the same hardness as rubies and sapphires; but these specimens were small, and cost very much more to produce than their commercial value.