igneous

Obsidian

Obsidian (principally SiO2)

Hardness: 5-6 (Mohs scale); Color: Black, sometimes dark brown or green; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.60.

Hardness: 5-6 (Mohs scale)
Color: Black, sometimes dark brown or green
Luster: Vitreous/Glassy

Identified February 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in geologically young volcanic areas (Pliocene to Holocene) because it eventually devitrifies into crystals over millions of years.

Uses & applications

Used historically for sharp tools like arrowheads and knives. Modernly used in surgical scalpels, jewelry (cabochons and beads), and as a decorative stone in architecture and interior design.

Geological facts

Because it lack crystals, obsidian can produce edges thinner and sharper than high-quality steel scalpels. It was highly valued in Mayan and Aztec cultures as both a weapon material and a mirror surface.

Field identification & locations

Identify by its glass-like appearance, smooth curved conchoidal fractures, and extremely sharp edges. Commonly found in volcanic regions of Oregon, California, and Iceland. Collectors should look for unique variants like Snowflake or Mahogany 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