Opal is colourless when pure, but is nearly always more or less milky and opaque, or tinted various dull shades by ferric oxide, magnesia or, alumina. The so-called black opal is generally a dark grey or blue, and very rarely quite black. That the coloration is not due to ordinary absorption, but to the action of cracks in the stone, is shown by the fact that the transmitted light is complementary to the reflected light; the blue opal is, for instance, a yellow when held up so that light has passed through it. In many black opals the opalescent material occurs in far too tiny pieces to be cut separately, and the whole iron-stained matrix is cut and polished and sold under the name ‘opal-matrix.’ The reddish and orange-coloured stones known as fire-opal have pronounced colour and only slight milkiness; they display the customary opalescence in certain directions. These stones are often faceted, but otherwise opals are cut _en cabochon_, either flat or steep—generally the former in brooches and pendants, and the latter in rings. Opal is somewhat soft, varying from 5 to 6½ on Mohs’s scale, and is therefore easily scratched. The specific gravity ranges from 2·10 to 2·20, and the refractive index from 1·444 to 1·464, the refraction, of course, being always single. It is unwise to immerse opals in liquids on account of their porosity. The name opal comes to us through the Latin _opallus_, which was used for the same species as understood by the term at the present day, but the word has a far older origin, which has not been traced. The Romans also called the mineral _pæderos_, the Greek form of Cupid, a name applied to all rosy stones. The name cacholong, for the bluish-white porcelain variety, which is very porous and adheres to the tongue, is of Tartar origin; the stone is highly valued in the East.