igneous

Obsidian

Volcanic glass (Mainly SiO2)

Hardness: 5-5.5 (Mohs); Color: Black (most common), can be brown, green, or rainbow; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.6

Hardness: 5-5
Color: Black (most common), can be brown, green, or rainbow
Luster: Vitreous/Glassy

Identified February 5, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of silica-rich (felsic) lava from volcanic eruptions. The cooling happens so quickly that crystals do not have time to grow. Most specimens are relatively young geologically (Cenozoic era) because glass is unstable over long periods.

Uses & applications

Historically used for making sharp blades, arrowheads, and scrapers. Modern uses include luxury surgical scalpels (which are sharper than steel), decorative jewelry, countertops, and ornamental carvings.

Geological facts

Obsidian scales are so sharp that they can be thin enough to cut between animal cells. While usually black, microscopic inclusions of magnetite can create an 'Electric' or 'Rainbow' sheen. Despite its dark color, it is chemically more similar to light-colored granite/rhyolite than dark basalt.

Field identification & locations

Identify in the field by its distinct conchoidal (curved) fracture lines and extremely sharp edges. Found in volcanic regions like the American West (Glass Buttes, OR), Iceland, Mexico, and Japan. Collectors look for unique varieties like Snowflake, Mahogany, or Fire Obsidian.

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