Spiderweb 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.