igneous

Obsidian

Obsidian (Volcanic Glass, ~70% SiO2)

Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be grey or reddish; Luster: vitreous (glassy); Structure: amorphous (non-crystalline); Fracture: conchoidal (shell-like curves); Specific Gravity: 2.3-2.6

Hardness: 5-5
Color: typically jet black but can be grey or reddish
Luster: vitreous (glassy)

Identified March 2, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Usually found in active or recently active volcanic regions such as the American Pacific Northwest, Mexico, and Iceland. Most deposits are less than 20 million years old.

Uses & applications

Historically used for arrowheads and blades; currently used in high-end surgical scalpels, jewelry (cabochons), and as a decorative stone in architecture and landscaping.

Geological facts

Because it is a volcanic glass, obsidian is metastable and eventually devitrifies into quartz over millions of years. It produces edges much sharper than steel knives because its structure is molecularly smooth rather than crystalline.

Field identification & locations

Identify in the field by its distinct glass-like appearance, extremely sharp edges where broken, and conchoidal fracture pattern. Commonly found in rhyolitic volcanic flows in the Western USA, Japan, and Italy.

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