Volcanic rock volcanic rock they are selling. The sub-family of rocks that form from volcanic lava are called igneous volcanic rocks (to differentiate them from igneous rocks that form from magma below the surface, called igneous plutonic rocks). The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava-stream cools quickly, it can quickly freeze into a black glassy substance called obsidian. When filled with bubbles of gas, the same lava may form the spongy appearing pumice. Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite. The lavas, having
Obsidian manufacture cutting and piercing tools and it has been used experimentally as surgical scalpel blades. The translation into English of "Natural History" written by Pliny the Elder of Rome shows a few sentences on the subject of a volcanic glass called obsidian ("lapis obsidianus"), discovered in Ethiopia by Obsidius, a Roman explorer. Obsidian is the rock formed as a result of quickly cooled lava, which is the parent material. Extrusive formation of obsidian may occur when felsic lava cools rapidly at the edges of a felsic lava flow or volcanic dome or when lava cools during sudden contact with water