Obsidian doesn't look like your typical rock. In fact, it looks more like dark glass than a rock you'd pick up in your backyard. That's because obsidian is volcanic glass. It forms when volcanic lava cools so quickly, there's no time for crystals to grow. This gives the rock a black, smooth, glossy shine. Since obsidian forms from magma, it is an igneous rock.

Image credits: main image, courtesy of AMNH; Rosamond Kinzler: AMNH.