Volcanic Glass, and the Smoking Mirror

Black obsidian is not a mineral but a volcanic glass, formed when felsic lava cools too fast for crystals to grow. It has no cleavage and breaks in smooth conchoidal curves to an edge finer than surgical steel, which is why it was worked into blades, points and scrapers across the Palaeolithic world and traded over long distances; sourcing obsidian tools by their chemical signature is now a standard archaeological method.

Polished into a flat sheet it takes a dark reflection, and mirrors of obsidian had a genuine ritual role in Mesoamerica, where the god Tezcatlipoca, the name means smoking mirror, was associated with obsidian scrying. The obsidian mirror later owned by the Elizabethan magus John Dee, now in the British Museum, is an Aztec piece. Modern crystal writing treats it as a grounding and protective stone; that framing is twentieth-century, though the scrying use is old and documented.