igneous

Obsidian

Volcanic Glass (SiO2 + impurities)

Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey (can be green, brown, or red); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.

Hardness: 5-5
Color: Jet black to dark grey (can be green, brown, or red)
Luster: Vitreous/Glassy

Identified January 12, 2026More igneous →

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Physical properties

Formation & geological history

Formed by the rapid cooling of felsic (silica-rich) lava, which prevents crystals from growing. It is typically found in geologically young volcanic regions (Quaternary-Neogene) as it eventually hydrates and devitrifies into other minerals.

Uses & applications

Historically used for making extremely sharp blades, scrapers, and arrowheads. Modern usage includes decorative jewelry (cabochons), precision surgical scalpels, and home decor items.

Geological facts

Obsidian is technically a 'mineraloid' rather than a true mineral because it lacks a crystalline structure. Ancient civilizations like the Aztecs and Maya valued it as a status symbol and used it for ritual mirrors and weapons.

Field identification & locations

Identify in the field by its characteristic smooth, curved (conchoidal) fracture surfaces that resemble broken glass and its sharp edges. It is commonly found in volcanic areas such as the western United States (Oregon, California, Iceland, and Mexico). Collectors look for a smooth, matte exterior often weathered into a 'desert varnish' or river-worn cobble shape.

Other igneous specimens

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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