igneous

Obsidian

Obsidian (volcanic glass), predominantly Silicon Dioxide (SiO2)

Hardness: 5-5.5 on Mohs scale; Color: typically black but can be brown, red (mahogany), or green; Luster: vitreous (glassy); Crystal structure: amorphous (none); Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.35-2.6

Hardness: 5-5
Color: typically black but can be brown, red (mahogany), or green
Luster: vitreous (glassy)

Identified March 20, 2026More igneous →

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Physical properties

Formation & geological history

Formed from rapid cooling of felsic lava, preventing crystal growth. Usually found in margin of rhyolitic lava flows. Most geologically recent (rarely older than a few million years due to devitrification).

Uses & applications

Used for surgical scalpel blades, decorative jewelry, healing crystals, and historically for sharp weapons and tools (arrowheads).

Geological facts

Obsidian is technically a glass rather than a true mineral because it lacks a crystalline structure. It was highly prized in pre-Columbian Mesoamerica for ritual knives and mirrors.

Field identification & locations

Identify by its extremely smooth, glassy appearance and sharp, curved (conchoidal) edges when broken. Found in volcanic regions like the American West (Glass Buttes, OR), Iceland, and Mexico.

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral