An opaque glass/stone formed as lava from volcanoes cools, which can be found just about anywhere that there has ever been volcanic activity. Most commonly solid black, but it has been found in gray, brown, blue, teal, a variety from Mexico called "rainbow obsidian," and a variety with bands of color from the San Carlos Apache Reservation in Arizona which is nicknamed "the tears of the Apache."

Obsidian is often heated before being sold, to try and get a deeper color. There are also products such as "Mount St. Helens stone," which are essentially synthetic obsidian made from volcanic ash which didn't naturally cool down into natural obsidian.