igneous

Obsidian

Volcanic Glass (SiO2, usually 70% or more)

Hardness: 5-5.5 on Mohs scale; Color: Black, sometimes dark green or brown; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Hardness: 5-5
Color: Black, sometimes dark green or brown
Luster: Vitreous (glassy)

Identified February 6, 2026More igneous →

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Physical properties

Formation & geological history

Formed from rapid cooling of felsic lava, which prevents crystal growth. It is relatively young geologically because it eventually devitrifies into crystalline rock; most specimens are from the Pliocene to Holocene epochs.

Uses & applications

Used for making surgical scalpels due to its edge sharpness, ornamental jewelry, decorative carvings, and prehistoric tools like arrowheads and knives.

Geological facts

Obsidian fractures with extremely sharp edges—thinner and sharper than steel scalpels. Ancient Aztecs used obsidian 'macuahuitl' clubs as effective weapons against the Spanish.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fractures and smooth, glassy surface. Commonly found in volcanic regions like the American West (Oregon, California), Mexico, and Iceland.

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

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral