Mahogany Obsidian

As in other streaked obsidian, the streaks form because molten obsidian is so viscous that magmas of different composition do not mix well as the lava slowly flows, leaving distinct layers of color. Oregon, USA, is a noted source; Glass Buttes in central Oregon yields mahogany obsidian.

Natural obsidian may contain crystallites, straight banding or needles, and it can react moderately to very strongly to a magnet; these features help separate it from artificial glass, although none is a definitive test. Jet, chalcedony and black tourmaline (schorl) can look similar, and a refractive index test separates them. Brightly colored or water-clear "obsidian" is often man-made glass.

Obsidian is brittle and chips or breaks easily on impact, and with a hardness of about 5 to 5.5 it scratches easily, so it suits earrings, pendants and brooches better than rings or bracelets. Household dust (hardness about 7) is harder than obsidian, so wipe it with a lint-free cloth instead of scrubbing it dry. Inclusions can expand when heated, so avoid mechanical cleaning systems and use mild soap, a soft brush and warm water.

Obsidian in general has been worked since the Stone Age: it breaks with a conchoidal fracture into very sharp edges, so it was made into knives, arrowheads, spear points and scrapers and was traded over distances of up to about a thousand miles. Ancient people also polished it into mirrors. According to the Roman naturalist Pliny the Elder, the name comes from Obsius, who found such a stone in Ethiopia.