igneous
Obsidian
Obsidian (Volcanic Glass, SiO2)
Hardness: 5-5.5 on Mohs scale; Color: typically black (but can be brown, green, or banded); Luster: vitreous (glassy); Crystal Structure: amorphous (non-crystalline); Cleavage: none, exhibits conchoidal fracture; Specific Gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- typically black (but can be brown, green, or banded)
- Luster
- vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically black (but can be brown, green, or banded); Luster: vitreous (glassy); Crystal Structure: amorphous (non-crystalline); Cleavage: none, exhibits conchoidal fracture; Specific Gravity: 2.35-2.6.
Formation & geological history
Formed through the rapid cooling of felsic (silica-rich) lava, which prevents the growth of mineral crystals. It is most commonly found in volcanic areas of recent geological age (Tertiary and Quaternary).
Uses & applications
Historically used for making sharp tools like arrowheads and knives; currently used as a gemstone for jewelry, in scalpel blades for delicate surgery, and for ornamental or decorative purposes.
Geological facts
Because it is a glass and lacks a stable crystalline structure, obsidian is metastable and eventually devitrifies into microscopic mineral crystals over millions of years. This is why very old (Precambrian) obsidian is rarely found.
Field identification & locations
Identify in the field by its intense glassy luster and curved, shell-like (conchoidal) fracture surfaces that produce extremely sharp edges. Commonly found in Oregon, California, and Mexico.
