igneous
Obsidian with Slate/Tuff
Volcanic glass (SiO2) with associated pyroclastic rock
Hardness: 5-5.5 (Mohs scale); Color: Black to dark grey; Luster: Vitreous to dull; Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Black to dark grey
- Luster
- Vitreous to dull
Identified More igneous →
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black to dark grey; Luster: Vitreous to dull; Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed by the rapid cooling of silica-rich lava that failed to crystallize. This specimen shows a mix of massive obsidian and a duller, lithic volcanic crust or ash layer, typically from the Tertiary or Quaternary periods.
Uses & applications
Used historically for making sharp cutting tools (scalpels, arrowheads) and currently for jewelry, countertops, and metaphysical collecting.
Geological facts
Obsidian was one of the first materials traded over long distances by prehistoric humans. It is technically a glass, not a mineral, because it lacks a crystalline structure.
Field identification & locations
Identify in the field by its characteristic curved, shell-like (conchoidal) fractures and sharp edges. Common in volcanic regions such as Iceland, Mexico, and the western United States.
