igneous

Obsidian

Obsidian (Volcanic Glass)

Hardness: 5-6 (Mohs scale), Color: Jet black to dark gray, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6

Identified May 7, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs scale), Color: Jet black to dark gray, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6

Formation & geological history

Formed from rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Usually found in volcanic regions from the Quaternary period to relatively recent eruptions because glass eventually devitrifies.

Uses & applications

Used historically for sharp edges, tools, and arrowheads. Modern uses include surgical scalpels, decorative jewelry, and collectible specimens.

Geological facts

Because it has no crystal structure, obsidian can be fractured to a thickness of only a few molecules, making it sharper than even high-quality steel scalpels. Ancient Aztecs used it for sacrificial knives and mirrors.

Field identification & locations

Identify by its glass-like texture and characteristic conchoidal (shell-like) fracture patterns. Commonly found in places like Oregon, Iceland, and Mexico. Collectors look for unique varieties like Snowflake or Mahogany obsidian.

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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