Obsidian

hand Sculpted in Zbrush, textured in substance Painter
suitable for animation, low poly asset with complete map textures

Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. The specimen shown above is about two inches (five centimeters) across.

Igneous rocks are formed from the solidification of molten rock material. There are two basic types. Intrusive igneous rocks & Extrusive igneous rocks. Obsidian is example of extrusive igneous.(http://geology.com/rocks/igneous-rocks.shtml)