Obsidian, or volcanic glass, is a volcanic rock with a siliceous composition and vitreous structure that was formed millions of years ago. Obsidian does not have the crystalline structure of a mineral because lava cools so fast that it does not have time to form crystals.
Obsidian is usually black, but it can also be brown, purple or green. Obsidian is found in special variations: the stone with white spots resembling snowflakes is snowdobsidian or lumiobsidian (white spots indicate the onset of crystallization), the brown-black stone is called Mahogany obsidian and the rounded pieces of volcanic glass are called apachis. The most valuable is considered to be the rainbow obsidian, which shines in purple, green and gold tones.
Common locations: United States, Mexico, Japan, Liapri Islands in Europe