igneous
Obsidian
Volcanic glass (SiO2 - Silicon Dioxide with impurities)
Hardness: 5–5.5 Mohs scale; Color: typically black but can be green, brown, or reddish; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
- Hardness
- 5–5
- Color
- typically black but can be green, brown, or reddish
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 Mohs scale; Color: typically black but can be green, brown, or reddish; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
Formation & geological history
Formed when felsic lava (rich in silica) cools so rapidly that crystals do not have time to grow. It is common in areas with geologically recent volcanic activity, dating back from the Pliocene to the Holocene epochs.
Uses & applications
Historically used for making sharp tools like arrowheads and knives. Today used in ornamental jewelry, as a gemstone, for manufacturing surgical scalpels, and as decorative stone in architecture.
Geological facts
Because it is glass and not a crystal, it is metastable and eventually devitrifies into quartz and feldspar over millions of years. This is why very old obsidian (pre-Cretaceous) is extremely rare.
Field identification & locations
Identify by its characteristic conchoidal (curved/shell-like) fracture edges which are razor-sharp. Commonly found in volcanic regions like the American West (Iceland, Mexico, Japan). Collectors look for translucent edges when held to the light.
