The inferior hardness and lower specific gravity of the Spinelle, are also very decided marks of distinction, and are easily applicable to the discriminating the two gems when cut and polished.
The Spinelle receives its color from the oxide of chrome, and the oriental ruby from oxide of iron.
From the red topaz, the Spinelle may be distinguished by its very feeble electricity; and from the garnet by its superior hardness and lower specific gravity.
THE OPAL.
MANY impositions have been practised in forming imitations, or enhancing the effect, of the Opal. The first I shall describe (if indeed that can be called an imposition, which, like the shades of a picture, tends to display to greater advantage its rich and glowing colors) is effected by warming the stone, and immersing it in oil or grease, which is afterwards burnt off. The rents which had absorbed the grease, by this means become dark, and agreeably contrast with the beautiful iridescence of the stone, which now assumes the name of the black or green opal.
