Obsidian Aztec Style Carving from Mexico done in the Aztec style. Obsidian is a naturally occurring volcanic glass that forms when lava cools rapidly with minimal crystal growth. Obsidian is technically not a true rock because it’s not made of mineral crystals, but rather a congealed liquid with some microscopic mineral crystals and impurities. Obsidian is usually black, sometimes green or brown. This sculpture is mainly black, with flecks of golden brown.