Obsidian is a black volcanic glass with a rhyolitic composition. Obsidian has no crystal structure and has a conchoidal fracture which gives very sharp edges- like glass. It is formed when rhyolite lava cools so rapidly that crystals cannot form. Rhyolite lava is rich in silica and would normally appear light in colour . A variant is snowflake obsidian which has a white "snowflake" like pattern. This where the glassy obsidian has devitrified, the Si O2 glass has crystallised into cristabolite.