Obsidian is a naturally occurring volcanic glass that forms when lava cools so rapidly that crystals don't have time to grow. Instead of the rough, grainy texture of most rocks, obsidian is smooth and shiny, typically black or very dark, with sharp edges when broken.

Ancient peoples prized obsidian for making cutting tools, arrowheads, and knife blades. When obsidian breaks, it can create edges sharper than even modern steel surgical scalpels. Archaeologists have found obsidian tools and weapons at sites thousands of years old, sometimes hundreds of miles from the nearest volcano, showing how valuable people considered this material.

The Aztecs and other Mesoamerican civilizations crafted obsidian into weapons and used polished obsidian as mirrors. Some obsidian contains tiny gas bubbles or mineral inclusions that create interesting patterns: snowflake obsidian has white spots, mahogany obsidian shows reddish swirls, and rainbow obsidian displays colorful sheens when light hits it just right.

Today, some surgeons use obsidian scalpel blades for delicate procedures because they can be made sharper than steel. Rock collectors and jewelry makers also value obsidian for its glassy beauty and the way it can be shaped and polished.