igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5; Color: Jet black to dark grey; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black to dark grey
- Luster
- Vitreous (glassy)
Identified More igneous →
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Physical properties
Hardness: 5-5.5; Color: Jet black to dark grey; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
Formation & geological history
Formed by the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. It is typically geologically young because it eventually crystallizes or hydrates into perlite.
Uses & applications
Historically used for knapping sharp tools and weapons; currently used in jewelry (gemstones), high-end surgical scalpels, and home decor.
Geological facts
Unlike most rocks, obsidian is not made of minerals but is technically a mineraloid. It can be found with white needle-like inclusions of cristobalite known as 'snowflake obsidian'.
Field identification & locations
Identify by its characteristic glassy texture and curved, razor-sharp conchoidal fractures. Commonly found in volcanic regions such as the Cascades (USA), Iceland, Mexico, and the Lipari Islands.
