Igneous
Obsidian
Volcanic Glass (Amorphous SiO2-rich glass)
Hardness: 5–5.5; Color: Black, sometimes translucent at edges; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5–5
- Color
- Black, sometimes translucent at edges
- Luster
- Vitreous (glassy)
Identified More igneous →
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Physical properties
Hardness: 5–5.5; Color: Black, sometimes translucent at edges; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. It is typically found in geologically recent volcanic areas (Cenozoic era).
Uses & applications
Used historically for sharp cutting tools such as arrowheads and knives. Today used for scalpel blades in heart surgery, ornamental jewelry, and gemstone sculpting.
Geological facts
Because it is a glass and lacks a crystalline structure, obsidian is metastable and eventually 'devitrifies' over millions of years, often turning into perlite or showing white snowflake-like inclusions of cristobalite.
Field identification & locations
Identify by its extremely smooth, curved (conchoidal) fracture surfaces and sharp edges. Common in volcanic regions such as the American Northwest, Iceland, and Mexico.
