igneous

Obsidian

Obsidian (volcanic glass), primarily SiO2 (silicon dioxide)

Hardness: 5-5.5 (Mohs scale). Color: Typically jet black, but can be dark green, brown, or gray. Luster: Vitreous/glassy. Crystal Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.3-2.6.

Hardness: 5-5
Color: Typically jet black, but can be dark green, brown, or gray
Luster: Vitreous/glassy

Identified March 28, 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. It is found in geologically young volcanic areas, mostly from the Pliocene to the Holocene era, as it devitrifies over millions of years.

Uses & applications

Historically used for arrowheads and knives. Today, it is used for surgical scalpel blades, decorative jewelry, cabochons, and as a semi-precious gemstone for collectors.

Geological facts

Obsidian breaks with a conchoidal fracture, creating edges that are thinner and sharper than high-quality steel scalpels. It is technically a mineraloid because it lacks a crystalline structure.

Field identification & locations

Identify in the field by its smooth, glassy surface and razor-sharp edges if broken. Commonly found in volcanic regions like the American West (Iceland, Mexico, Japan). Collectors look for 'Snowflake' or 'Rainbow' varieties.

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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