Rose-colored spinels are known as balas-rubies, pale-blue spinels as sapphirines, and the hyacinth-red, yellowish-red, and orange-yellow spinels are called rubicelles.
All these different-colored spinels, if pure and of great brilliancy, are valuable as gem stones, being only surpassed in hardness and brilliancy by the diamond and corundums.
The white spinel, which is seldom found, is sometimes confounded with the diamond, having the same specific gravity and single refraction, but as it lacks the fire and is easily scratched by the diamond, the danger of mistaking one for the other is slight. Burnt amethyst, which often resembles the spinel, is lighter and softer, while burnt topaz, although it is identical with the spinel in hardness, is somewhat lighter and possesses remarkable electric powers, becoming electric by either rubbing, heating, or pressure, and retaining electricity for upwards of twenty-four hours.
The zircon is easily distinguished from the spinel because of its much greater specific gravity. It is also doubly refractive and softer.
Garnets are softer, lack the play of color and brilliancy, and fuse easily into a light-brown or black glass.
Beryl.
The beryl is a mineral belonging to the primitive formation, and is found in quartz veins and granite.
It crystallizes in six-sided prisms and is composed largely of silica, the third most common of earth’s productions. The beryl is 7.5 to 8 in hardness, scratching quartz, but is scratched by topaz.
The specific gravity is 2.67 to 2.73, making it one of the light minerals. Its lustre is vitreous and refraction double to a slight degree; its cleavage is imperfectly basal, and it becomes electric by rubbing.
Acids do not attack the beryl, but it melts with borax and is soluble in salts of phosphorus.