OBSIDIAN, a volcanic glass, said to have been named after its discoverer Obsidins. It is usually of black, brown, or grey colour, and in some varieties banded or striated. The colour is generally due not so much to the vitreous mass itself as to the presence of minute foreign bodies or microlites, which in some cases appear to be incipient crystals of felspar, while in others they probably consist of such minerals as augite, hornblende, biotite, and magnetite. These microscopic enclosures are occasionally arranged in curved lines, producing beautiful examples of fluidal structure (see GEOLOGY, vol. x. p. 230, fig. 2). Spherulites are not uncommon in obsidian, and are sometimes sufficiently large to impart a distinctly globular structure to the stone. Other varieties are rich in microscopic pores, or may even present to the naked eye a vesicular texture. It is notable that certain kinds of obsidian possess a peculiar metallic sheen, which has been attributed by Professor Zirkel to the presence of minute ovoid enclosures, and not to a porous structure, as had been previously suspected. There can be no doubt that obsidian has been formed by the rapid cooling of a felspathic lava. It is found chiefly in Iceland, the Lipari Islands, Melos and other isles of the Greek archipelago, the Caucasus, Siberia, Mexico, Peru, and New Zealand.

Obsidian breaks with a beautifully conchoidal fracture, yielding sharp-edged,fragments, which have been largely used in various putt of the world as arrow-points, spearheads, and rude knives. For these purposes it was extensively employed, under the name of itztli, by the ancient Mexicans, who quarried it at the Cerro de las Navajas, or " Hill of Knives," near the head-waters of the Great Barauca. Obsidian has also been used as a mirror, - a purpose for which its strong lustre has recommended it. By the ancient Greeks and Romans it was worked as a gem-stone ; and, in consequence of its having been often imitated in black glass, there arose among collectors of gems in the last century the curious practice of calling all antique pastes "obsidians." Even at the present day the bottle-green varieties of obsidian are occasionally cut and polished as ornamental stones. They bear some resemblance to peridotes and tourmalines, but are deficient in hardness.