Obsidian has been use since the stone age owing to the fact that like flint it can be fashioned into arrowheads. Obsidian is in fact a type of glass formed in volcanic erruptions and so, strictly speaking, is not a mineral. Like all glass obsidian breaks with a characteristic fracture. In ancient times it was also used to make mirrors as it can be polished to a high sheen. Obsidian beads glint in the sun and often have bands of reflectvie minral running through the stone.