igneous

Obsidian

Volcanic glass (high silica content based on SiO2)

Hardness: 5-5.5 (Mohs); Color: Raven black, often translucent on thin edges; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Hardness: 5-5
Color: Raven black, often translucent on thin edges
Luster: Vitreous/glassy

Identified March 16, 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. Most specimens belong to the Cenozoic era, as volcanic glass eventually devitrifies into quartz and feldspar over millions of years.

Uses & applications

Used in the manufacture of surgical scalpels (blades are thinner than steel), jewelry/cabling, and various decorative items. Historically vital for making arrowheads and cutting tools.

Geological facts

Obsidian is so sharp at the molecular level that it can be used for heart surgery to minimize scarring. Unlike most rocks, it is technically an 'extra-viscous liquid' rather than a true solid due to its lack of internal crystalline structure.

Field identification & locations

Identify by its distinctive conchoidal fracture (curved, shell-like breaks) and razor-sharp edges. Common in volcanic regions with high silica rhyolitic flows, such as Oregon, California, Iceland, and Mexico.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral