Obsidian is usually jet black, however, it can also be green, brown, red, blue, yellow or orange. The different colors are thought to occur due to inclusions or trace elements. It is an igneous rock that results from lava cooling rapidly and creating volcanic glass.
Sometimes two colors of obsidian may be swirled together, commonly black and brown, which is called "mahogany obsidian." Obsidian has been used to make tools such as knives, spear points and arrowheads. In modern surgery thin blades of obsidian are placed in some surgical scalpels. Obsidian is a popular jewelry stone. Its Mohs hardness scale number is 5.5 which makes it fairly easy to scratch.