Obsidian is a type of naturally occuring glass, produced from volcanoes when the right kind of lava cools rapidly, e.g., by flowing into a body of water. It consists mainly of SiO2 (silicon dioxide). It is mineral-like, but not a true mineral because it's not crystalline. It's sometimes classified instead as a mineraloid.
Its colour varies depending on the presence of impurities. It may contain patterns of air bubbles remaining from the lava flow.
It was highly valued in certain stone age cultures because, like glass, it can be fractured to produce sharp blades or arrow heads. It may also have been polished to create early mirrors.