igneous
Black Obsidian
Volcanic Glass (SiO2 with impurities)
Hardness: 5 to 5.5 on Mohs scale. Color: Deep solid black. Luster: Vitreous to glassy. Structure: Amorphous (non-crystalline). Cleavage: None (features conchoidal fracture).
- Hardness
- 5 to 5
- Color
- Deep solid black
- Luster
- Vitreous to glassy
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Physical properties
Hardness: 5 to 5.5 on Mohs scale. Color: Deep solid black. Luster: Vitreous to glassy. Structure: Amorphous (non-crystalline). Cleavage: None (features conchoidal fracture).
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in volcanic environments worldwide, particularly in areas with recent geologically young activity like Mexico, the western US, and Iceland.
Uses & applications
Used for lapidary work, decorative cabochons, polished palm stones, carved figurines, and historically for ultra-sharp blades and arrowheads.
Geological facts
Obsidian is technically not a true mineral because it is an amorphous glass rather than a crystal. Its fracture edges can be thinner and sharper than high-grade surgical steel scalpel blades.
Field identification & locations
Identified by its jet-black color, extremely smooth glassy surface when polished, and distinct curved conchoidal fracture lines if chipped. Found near rhyolitic volcanic deposits.
