igneous

Obsidian

Obsidian (Volcanic Glass)

Hardness: 5-5.5 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Hardness: 5-5
Color: Black to dark brown
Luster: Vitreous/Glassy

Identified May 16, 2026More igneous →

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Physical properties

Hardness: 5-5.5 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Formation & geological history

Formed from the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. It is relatively young geologically as it eventually devitrifies into crystals.

Uses & applications

Historically used for cutting tools, arrowheads, and mirrors. Today used in scalpel blades, jewelry, and as a collector's specimen.

Geological facts

Obsidian fractures with extremely sharp edges, sometimes thinner than a molecule. In ancient cultures, it was a highly traded commodity for making sharp weapons and tools.

Field identification & locations

Identify by its glassy texture and conchoidal (shell-like) fracture patterns. Frequently found in volcanic areas like the Western USA, Mexico, and Iceland.

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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