Obsidian
Obsidian is a glassy volcanic rock. Magmas that cool
rapidly, or that were erupted at relatively low
temperatures, may not be able to form crystals before they
solidify, and so remain in the form of a silicate glass. A
broken piece of obsidian is easy to identify: it looks like
dark bottle glass, with sharp edges and curved fracture
surfaces, well shown in the photographs. Obsidian is usually
of acid composition, despite its dark colour. Basic magmas
are hotter and more "runny", and form crystals more easily.