Obsidian is not a stone.

It is glass. When lava flows from a volcanic vent and cools fast enough, the minerals have no time to crystallise and solidify into an amorphous mass. That is obsidian. Its composition is similar to granite — both are silica-rich acidic magma — but granite cools slowly underground, allowing crystals to develop fully. Obsidian cools rapidly at the surface, producing no crystals, only glass.

Glass fractures differently from crystal. Crystal breaks along cleavage planes, the fracture following fixed directions. Glass fractures conchoidally — the break is curved, the edge extremely thin, capable of becoming arbitrarily sharp. An obsidian blade measured under an electron microscope can reach a thickness of three nanometres, tens of times thinner than a modern stainless steel surgical blade. Some surgeons still use obsidian blades for certain delicate procedures today, because the incisions they produce are cleaner than metal and heal faster.

The Aztecs knew this.

They used obsidian for arrowheads, spear points, and daggers. They also used it to make the macuahuitl — a wooden war club with obsidian blades set into both sides, the primary weapon of Aztec warriors. Spanish conquistador chronicles record that a macuahuitl strike could sever a horse’s head. The claim may be exaggerated. The cutting capacity of an obsidian edge is not.

The Aztecs also made mirrors from obsidian. A polished obsidian surface reflects light, though less completely than metal — the image in the mirror is dark and indistinct. Aztec priests used obsidian mirrors for divination, reading the blurred reflections as divine messages or omens of the future. One of the most important Aztec deities, Tezcatlipoca, had a name meaning Smoking Mirror. His emblem was an obsidian mirror said to reveal the sins of every person in the world.

When the Spanish took Tenochtitlan in 1521 and the Aztec empire fell, obsidian mirrors entered Europe among the spoils and tribute. John Dee, court astrologer to Elizabeth I, owned one. He reportedly used it to communicate with angels. The mirror is now in the British Museum, catalogued as AOA+9663.

The protective function attributed to obsidian developed independently across multiple cultures, but the underlying logic is consistent: black absorbs negative energy, the glassy surface reflects malevolent forces, the sharp edge cuts through what should not be there.

In Mesoamerican tradition, obsidian was placed at doorways or beneath beds to prevent evil spirits from entering. This practice is documented in Olmec culture, predating the Aztecs by nearly two thousand years. When obsidian entered China, it was absorbed into feng shui and warding systems — black corresponds to water, water controls fire, fire represents malevolent energy, therefore obsidian suppresses it. This interpretation was constructed after the fact and has no connection to the original Mesoamerican logic. Both systems arrived at similar conclusions.

Rainbow sheen obsidian is considered more potent within this framework, because the iridescent colours are said to represent the five elements in balance. This claim has no basis in ancient texts; it is a narrative that developed in the modern market. The rainbow sheen itself is a real physical phenomenon. As lava cools, gases that cannot fully escape form nanoscale bubble layers within the obsidian. Light entering the stone reflects between the upper and lower surfaces of these layers; the two reflected beams overlap and interfere, producing iridescent colour. The thickness of the bubble layer determines the wavelength of the interference — that is, the colour of the rainbow. Different obsidian deposits produce bubble layers of different thicknesses, and different colours.