Obsidian is one of the sharpest naturally occurring materials. When broken, it has razor-sharp edges.
Obsidian breaks with a conchoidal fracture, a characteristic of most types of glass.
Although black is the best-known variety, obsidian occurs in several colours and with various inclusions and sheens.
Obsidian is a non-crystalline solid that forms from rapidly cooling lava. Classed as a rock, not a mineral, it's correctly known as a mineraloid.
An advisor to Queen Elizabeth I used a black obsidian mirror, believing it enabled him to communicate with the spirit world.
The mirror, housed in the British Museum, was designed by the Aztecs in Mexico.
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