Obsidian is a naturally occurring, volcanic glass formed when silica-rich lava cools rapidly. This rapid cooling is why obsidian is not crystalline, and therefore not a true mineral, though it is sometimes classified as a mineraloid. Unlike intrusive minerals that take thousands of years to form from magma cooling inside the Earth, obsidian is an extrusive rock, which means it forms after being erupted.
It typically occurs black, but if other minerals are present during formation they can sometimes add color to the glass. In the case of mahogany obsidian, iron-rich hematite and magnetite add reddish streaks throughout which can resemble mahogany timber. Obsidian can have very sharp edges; in fact, obsidian blades can be made much sharper than steel surgical scalpels.
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