igneous

Obsidian

Amorphous Volcanic Glass (primarily SiO2)

Hardness: 5-5.5 (Mohs); Color: Jet black to dark gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6

Hardness: 5-5
Color: Jet black to dark gray
Luster: Vitreous (glassy)

Identified March 28, 2026More igneous →

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Physical properties

Hardness: 5-5.5 (Mohs); Color: Jet black to dark gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6

Formation & geological history

Formed from rapid cooling of felsic (silica-rich) lava, preventing crystal growth. Usually found in volcanic margins and dated from the Cenozoic era to recent times.

Uses & applications

Used in gemstone manufacturing, knapping for surgical scalpels, decorative ornaments, and abrasive grit. Highly valued in historical toolmaking.

Geological facts

Because it is chemically unstable over long geological time periods, obsidian older than a few million years is rare, as it eventually devitrifies into crystalline rock.

Field identification & locations

Identify by its sharp edges and smooth, curved (conchoidal) fracture surfaces like broken glass. Commonly found in Oregon, Iceland, and Mexico. Collectors should look for unique internal patterns like 'snowflake' or 'rainbow' sheen.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous