Obsidian is not a crystal in the traditional sense — it is volcanic glass, formed when molten lava cools so rapidly that crystals do not have time to grow. The result is a smooth, glossy, usually black stone that has been used by humans for at least 700,000 years.
Obsidian can be polished to a mirror-like finish, and ancient civilizations used it for scrying mirrors, arrowheads, and surgical tools. The Aztecs called it *itztli* ("the divine stone") and used it for both weapons and spiritual divination.
Today, obsidian is one of the most popular protection stones in crystal healing — worn by people who want to shield themselves from negative energy.