Obsidian is an igneous rock, a mix of cryptocrystalline grains of silica minerals in a glass-like suspension. It is a natural occurrence of volcanic glass created when lava from a volcano cools rapidly with minimal crystal growth. 
 
Since Obsidian is a natural glass it may have razor-sharp edges that can easily cut skin and flesh if broken. Handle with care. Do not grind dry since long-term exposure to finely ground powder may lead to silicosis. 
 
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* Sizes, shapes and colors my vary due to being a natural stone, being hand-carved and/or difference in computer monitors.