Obsidian

Obsidian is a natural volcanic glass: lava, usually felsic and rich in silica, that cooled so quickly that almost no crystals had time to grow. Because it is a glass, and its make-up varies, it is not a true mineral; Wikipedia calls it an igneous rock that is sometimes classed as a mineraloid. It is mostly silicon dioxide, often 70 per cent or more by weight. Most pieces are black or dark brown from iron and other impurities, and white cristobalite clusters, trapped gas bubbles or tiny inclusions can give it snowflake spots, a golden sheen or rainbow stripes. It turns up near volcanoes that have erupted rhyolite, from Mexico and the western United States to Italy, Turkey, Japan and New Zealand. It breaks with very sharp edges, so people knapped it into blades and arrowheads from the Stone Age on, and polished it into early mirrors; John Dee owned an obsidian mirror brought from Mexico. Cunningham gives it to Saturn and fire, links it with Tezcatlipoca, and uses it for protection, grounding, divination and peace, scrying with polished spheres. Hall calls it a fast, powerful and strongly protective stone that grounds, drags unwelcome truths into view and helps a person face their shadow side.