Igneous

Obsidian

Obsidian (Volcanic Glass)

Hardness: 5-6 (Mohs scale); Color: Typically black, can be green/brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35-2.6.

Hardness: 5-6 (Mohs scale)
Color: Typically black, can be green/brown
Luster: Vitreous (glassy)

Identified February 19, 2026More igneous →

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Physical properties

Formation & geological history

Forms from rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. Usually found in geologically young volcanic regions (Quaternary or Tertiary periods).

Uses & applications

Historically used for weapons/tools; modern use includes surgical scalpel blades, jewelry, and ornamental carvings.

Geological facts

Because it is so sharp (down to a molecular level), obsidian scalpels are often thinner and sharper than high-quality steel scalpels. It is technically a mineraloid rather than a mineral.

Field identification & locations

Identify by its distinctive curved, shell-like (conchoidal) fractures and translucent edges. Common in volcanic areas like Iceland, Mexico, and the Western USA (Oregon/California).

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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