Igneous
Obsidian
Volcanic Glass (SiO2 rich)
Hardness: 5-5.5 (Mohs scale); Color: Typically deep black (sometimes greenish, mahogany, or rainbow); Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like); Specific Gravity: 2.3-2.6
- Hardness
- 5-5
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Typically deep black (sometimes greenish, mahogany, or rainbow); Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like); Specific Gravity: 2.3-2.6
Formation & geological history
Formed from extrusive volcanic activity when highly viscous, silica-rich lava cools extremely rapidly, preventing the growth of mineral crystals. It is geologically young as it eventually devitrifies into fine-grained minerals.
Uses & applications
Historically used for weapons (arrowheads, knives) due to its razor-sharp edges. Currently used in specialized medical scalpels, jewelry (beads, cabochons), ornamental carvings, and for meditation/metaphysical collecting.
Geological facts
Obsidian can be fractured to reveal an edge that is much thinner and sharper than a high-quality steel scalpel. Because it lacks crystal structure, it is technically a 'mineraloid' rather than a true mineral.
Field identification & locations
Identify in the field by its distinct conchoidal fracture (curved, glass-like breaks) and lack of visible grains. Commonly found in areas with geologically recent volcanic activity, such as the Western USA (Oregon, California), Mexico, and Iceland.
