igneous
Obsidian with Perlitic Crust
Volcanic Glass (SiO2)
Hardness: 5-6 (Mohs); Color: Jet black to greyish-black; Luster: Vitreous (glassy); Structure: Amorphous; Fracture: Conchoidal; Specific Gravity: 2.4
- Hardness
- 5-6 (Mohs)
- Color
- Jet black to greyish-black
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-6 (Mohs); Color: Jet black to greyish-black; Luster: Vitreous (glassy); Structure: Amorphous; Fracture: Conchoidal; Specific Gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich (rhyolitic) lava, preventing crystal growth. This specimen shows a weathered exterior (perlitic/pumiceous crust) with a glassy interior. Geologically associated with the Trans-Mexican Volcanic Belt.
Uses & applications
Used historically for cutting tools and jewelry; currently popular in lapidary work, decorative objects, and for some high-precision surgical blades.
Geological facts
This specimen displays a combination of dark, glassy obsidian and a grey, more porous weathered layer. The region around 20.0N, 100.1W is part of the Mexican Cenozoic volcanic arc, which is world-famous for high-quality obsidian deposits used by Mesoamerican civilizations.
Field identification & locations
Identified by its sharp, glass-like fracture (conchoidal) and deep black color when chipped. Commonly found in volcanic domes or lava flows. In the field, look for 'glassy' pebbles within grey ash or rhyolite deposits.
