Obsidian is volcanic glass. It was a very important material in Ancient America, especially in Mesoamerica, where metal was introduced late. The first metal objects appear there in the second half of the first millennium AD. Obsidian can be used to make razor-sharp blades and other tools. The technique is similar to that of flint knives. A hard stone is struck at the nucleus so that a larger piece splits off. This is then further processed by removing more flakes. The Aztecs also used obsidian to make mirrors and jewellery.