Obsidian

Obsidian is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium.

It is commonly found within the margins of rhyolitic lava flows known as obsidian flows.
These flows have a high content of silica, giving them a high viscosity.
The high viscosity inhibits the diffusion of atoms through the lava, which inhibits the first step (nucleation) in the formation of mineral crystals.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY-SA 3.0

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