igneous

Obsidian

Volcanic glass (SiO2 - Silicon Dioxide with impurities)

Hardness: 5–6 Mohs; Color: Jet black to dark green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3–2.6.

Hardness: 5–6 Mohs
Color: Jet black to dark green
Luster: Vitreous (glassy)

Identified March 24, 2026More igneous →

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Physical properties

Hardness: 5–6 Mohs; Color: Jet black to dark green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3–2.6.

Formation & geological history

Formed from extrusive igneous activity when felsic lava (rich in silica) cools so rapidly that crystal growth is inhibited. It often forms at the edges of basaltic lava flows or when lava enters water.

Uses & applications

Historically used for blades, arrowheads, and mirrors. Modern uses include surgical scalpels, ornamental jewelry, carvings, and feng shui 'protective' stones.

Geological facts

Because it has no crystal structure, its edges can be thinned down to almost molecular thinness, making it sharper than high-quality steel scalpels. Famous locations include the Glass Buttes in Oregon and various sites across the Ring of Fire.

Field identification & locations

Identify by its characteristic conchoidal (shell-like) fracture, extremely smooth surface, and razor-sharp edges if broken. Common in Cenozoic volcanic regions; look for it in areas with rhyolitic eruptions.

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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)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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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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