igneous

Obsidian

Obsidian (Volcanic Glass; SiO2 content > 70%)

Hardness: 5–6 Mohs; Color: Jet black (though can be green, brown, or gray in thin sections); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.35–2.6

Hardness: 5–6 Mohs
Luster: Vitreous/Glassy

Identified March 16, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich (felsic) lava, prevents crystal growth. Usually found in volcanic regions of any geological age, though most specimens are relatively young because obsidian eventually devitrifies into crystalline rock.

Uses & applications

Used in ornamental jewelry, as a healing stone in lapidary, and in specialized surgical scalpels due to its ability to hold an edge sharper than steel. Historically used for weapons like arrowheads and knives.

Geological facts

Because it lacks a crystal structure, obsidian is technically a 'mineraloid' rather than a mineral. Native Americans traded it extensively across North America, and it was the material used for Aztec 'Macuahuitl' swords.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture, sharp edges, and glassy appearance. Common locations include the Western USA (Oregon, California), Iceland, Mexico, and Japan. Collectors look for translucency at the edges.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral