Obsidian is a type of naturally-occurring glass formed as an extrusive igneous rock. It is produced when felsic lava erupted from a volcano, cools rapidly through the glass transition temperature and freezes without sufficient time for Crystal growth. Because of the lack of Crystal structure, Obsidian blade edges can reach almost molecular thinness, leading to its ancient use of projectile points (arrow heads), and its modern use as surgical scalpel blades.
Rainbow Obsidian, also called Heaven Eye, is a black or deep brown Obsidian displaying iridescent bands of red, blue, gold, violet, or green when polished and exposed to bright light. The color layers are caused by the refraction of microscopic bubbles and inclusions such as Magnetite.