What is volcanic glass called?

Volcanic glass is typically called obsidian and is formed when viscous lava is rapidly cooled. This material contains between 65 to 80 percent silica and has a low water content. Obsidian is slightly more durable than window glass and possesses a glassy luster.

Typically, naturally produced obsidian will appear jet black in color. In some obsidian mixtures, the presence of iron oxide can cause it to appear brown or red. Should the obsidian cool with the presence of gas bubbles, its outer color takes on a golden sheen. Obsidian can be found in various locations, including the Obsidian Cliff in Yellowstone National Park or Mount Hekla in Iceland.