Rock cycle called intrusive or plutonic and cools very slowly, producing a coarse-grained texture such as the rock granite. As a result of volcanic activity, magma (which is called lava when it reaches Earth's surface) may cool very rapidly while being on the Earth's surface exposed to the atmosphere and are called extrusive or volcanic rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian, however the most common fine grained rock would be known as basalt. Any of the three main types of rocks (igneous, sedimentary, and
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