Obsidian is a naturally occurring volcanic glass formed when lava cools rapidly, preventing the formation of crystals. It is primarily composed of silicon dioxide (SiO₂), with trace amounts of other minerals. Obsidian is typically black, though it can also appear in shades of brown, green, or even rainbow-like iridescent hues due to mineral inclusions. It has a smooth, glassy texture and fractures with a sharp edge, which historically made it useful for tools and weapons. Obsidian has a Mohs hardness of 5 to 5.5, making it brittle and best suited for carvings or ornamental uses. Major sources include the United States, Mexico, Iceland, and Japan.