Obsidian is a glassy, silica-rich volcanic with a hardness of 5 to 5.5. It is an amorphous stone, meaning that its structure contains no regular geometric pattern. Because it breaks with a very sharp conchoidal fracture, it has been used for making arrowheads and other tools since Paleolithic times. The name Obsidian is derived from the name of the prominent ancient Roman, Obsius. Obsidian is most frequently black or very dark brown, but it can also be gray or green.