igneous

Obsidian

Volcanic Glass (SiO2 - Silicon Dioxide with impurities)

Hardness: 5–6 on Mohs scale; Color: typically black but can be dark green, brown, or grayish; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.4

Hardness: 5–6 on Mohs scale
Color: typically black but can be dark green, brown, or grayish
Luster: Vitreous (glassy)

Identified March 30, 2026More igneous →

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Physical properties

Formation & geological history

Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in many locations worldwide that have experienced rhyolitic eruptions, primarily during the Cenozoic era.

Uses & applications

Historically used for making sharp tools and weapons (arrowheads, blades). Today used in jewelry, as ornamental stones, and in specialized surgical scalpels due to its incredibly sharp edge.

Geological facts

Because it is a glass and lacks a crystal structure, obsidian is technically naturally occurring volcanic glass rather than a true mineral. It is often sharper than high-quality surgical steel on a microscopic level.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture patterns and extremely smooth, glassy surfaces. Common locations include the Western United States (Oregon, California), Mexico, Iceland, and Italy. Collectors look for unique varieties like Snowflake or Mahogany Obsidian.

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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