igneous

Obsidian

Obsidian (Volcanic Glass)

Hardness: 5-6 (Mohs); Color: Black, sometimes dark green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Hardness: 5-6 (Mohs)
Color: Black, sometimes dark green or brown
Luster: Vitreous/Glassy

Identified May 25, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs); Color: Black, sometimes dark green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Formation & geological history

Formed from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. Common in areas with recent volcanic activity (Cenozoic era).

Uses & applications

Historically used for arrowheads and scrapers; currently used for surgical scalpel blades, jewelry, and ornamental carvings.

Geological facts

Because it is a glass and not a crystal, obsidian is technically metastable and will eventually hydrate and turn into perlite over millions of years. It was highly prized in pre-Columbian Mesoamerica for tools and ritual objects.

Field identification & locations

Identify by its extremely smooth, glass-like texture and sharp, curved conchoidal fractures. Common in volcanic regions like the American West (Glass Buttes), Mexico, and Iceland.

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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