The basics
So what is obsidian, really?
Obsidian forms when lava cools too fast for crystals to grow. That makes it natural glass, not a mineral. Geologists call it a mineraloid.
Tiny bits inside change the look. Gas bubbles can give a gold or silver sheen, iron gives mahogany obsidian its red-brown, and microscopic inclusions make rainbow obsidian glow.
It breaks with sharp, curved edges, so people have shaped it into blades and arrowheads for thousands of years.


