igneous

Obsidian

Volcanic Glass (Primary composition: SiO2 - Silicon Dioxide)

Hardness: 5-5.5 (Mohs scale); Color: Black or deep brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6

Hardness: 5-5
Color: Black or deep brown
Luster: Vitreous (glassy)

Identified February 10, 2026More igneous →

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Physical properties

Hardness: 5-5.5 (Mohs scale); Color: Black or deep brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6

Formation & geological history

Formed from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. Found in geologically young volcanic areas (Cenozoic era) because it eventually devitrifies into crystalline material.

Uses & applications

Used historically for sharp tools and weapons; currently used in jewelry (cabochons), scalpel blades for delicate surgery, and as a decorative stone.

Geological facts

Obsidian is technically an igneous rock but behaves like a mineraloid. It breaks with extremely sharp edges—sometimes thinner than a human hair or a steel scalpel. A common variety called 'Apache Tears' are rounded nodules of obsidian found in the American Southwest.

Field identification & locations

Identify in the field by its characteristic conchoidal (curved, shell-like) fractures and glassy texture. Commonly found in volcanic fields such as Glass Buttes in Oregon, the Yellowstone region, and areas of Iceland and Mexico.

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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.

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Magnetite

Iron(II,III) oxide (Fe3O4)

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Blue Apatite

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

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Magnetite

Magnetite (Fe3O4)

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