What type of rock is obsidian?

Obsidian is igneous rock that forms when specific rock material cools so abruptly that atoms cannot arrange themselves into a crystalline structure. Obsidian is usually an extrusive rock that solidifies above the earth's surface. It can form in a plethora of cooling environments. Obsidian is most often black. It can sometimes be brown, tan or green. It is very rarely blue, red, orange or yellow.

Occasionally, two colors of obsidian will be swirled together in a single rock. For example, black, and brown obsidian can be swirled together, which is called mahogany obsidian. Obsidian is found in areas that demonstrate volcanic activity.