Obsidian

Obsidian, a natural glass—the vitreous condition of an acid lava. It is hard and brittle, with remarkably vitreous lustre, and perfectly conchoidal fracture, the edges of the fractures very sharp and cutting like glass. It varies from semi-transparency to translucency only on the edges. It is often black or very dark gray; sometimes green, red, brown, striped, or spotted; and sometimes chatoyant or avanturine. Some obsidians are rendered porphyritic with microscopic crystals of sanidine; others are often highly vesicular and plentifully charged with spherulites usually arranged in the line of lava-flow. The rock is usually rich in crystallites and microlites (the 'beginnings of crystallisation'), which are frequently arranged in parallel or undulating lines = 'fluxion-structure.' Steam- or vapour-pores of minute size occur abundantly in some obsidians. Obsidian is thus a kind of lava. It is capable of being polished, but is apt to break in the process. It is made into boxes, buttons, ear-drops, and other ornamental articles; and before the uses of the metals were well known it was employed, in different parts of the world, for making arrow and spear heads, knives, &c. It is found in Iceland, the Lipari Isles, Vesuvius, Sardinia, Hungary, Spain, Teneriffe, Mexico, South America, Madagascar, Siberia, &c. Black obsidian was used by the ancients for making mirrors, and for this purpose was brought to Rome from Ethiopia. It was used for the same purpose in Peru and Mexico. Mirrors of black obsidian are indeed still employed by artists. Chatoyant or avanturine obsidian is very beautiful when cut and polished, and ornaments made of it are sold at a comparatively high price.