igneous

Obsidian

Volcanic glass (SiO2 - Silicon Dioxide with impurities)

Hardness: 5–5.5 Mohs scale; Color: typically black but can be green, brown, or reddish; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.4

Hardness: 5–5
Color: typically black but can be green, brown, or reddish
Luster: Vitreous (glassy)

Identified March 16, 2026More igneous →

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Physical properties

Formation & geological history

Formed when felsic lava (rich in silica) cools so rapidly that crystals do not have time to grow. It is common in areas with geologically recent volcanic activity, dating back from the Pliocene to the Holocene epochs.

Uses & applications

Historically used for making sharp tools like arrowheads and knives. Today used in ornamental jewelry, as a gemstone, for manufacturing surgical scalpels, and as decorative stone in architecture.

Geological facts

Because it is glass and not a crystal, it is metastable and eventually devitrifies into quartz and feldspar over millions of years. This is why very old obsidian (pre-Cretaceous) is extremely rare.

Field identification & locations

Identify by its characteristic conchoidal (curved/shell-like) fracture edges which are razor-sharp. Commonly found in volcanic regions like the American West (Iceland, Mexico, Japan). Collectors look for translucent edges when held to the light.

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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)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral