Stone Type: Obsidian
Obsidian is a volcanic glass that forms when lava cools so rapidly that the atoms are unable to organise into crystals. Obsidian occurs from a specific form of lava and the composition is similar to rhyolite and granite. The glass can recrystallise over time, leading to patches of white crystobalite crystals within the glassy matrix.
An iridescent sheen can occur in obsidian caused by light reflecting from minute inclusions of minerals, debris, or gas. Most obsidian is black but it can occur in various shades of brown, tan, green, red, blue, or yellow, depending on trace elements. Obsidian is an excellent material for flaking, although it is more fragile than chert. The edge of an unmodified obsidian flake is sharper than a modern steel surgical scalpel.