California Rainbow Obsidian Is a Natural Wonder (Amazing Photos)

Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color. Obsidian with multicolored iridescence caused by inclusions of magnetite nanoparticles. Obsidian is often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow.

The ground at Needle Hill is absolutely littered with black and mahogany obsidian cobbles. photo: Scott's Rock & Gem