Igneous

Obsidian (specifically Rainbow or Silver Sheen Obsidian)

Volcanic Glass (predominantly SiO2)

Hardness: 5–5.5 Mohs scale; Color: Black with metallic silver or rainbow iridescence; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Specific Gravity: 2.35–2.6; Conchoidal fracture.

Hardness: 5–5
Color: Black with metallic silver or rainbow iridescence
Luster: Vitreous (glassy)

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed when extrusive felsic lava cools rapidly with minimal crystal growth. The 'sheen' or 'rainbow' effect is caused by microscopic inclusions of gas bubbles or minerals like magnetite or feldspar oriented along flow layers. Typically found in young volcanic regions (Cenozoic to Holocene).

Uses & applications

Used primarily in jewelry (beads, cabochons), ornamental carvings, and historically for sharp tools/scalpels due to its ability to hold an edge thinner than steel.

Geological facts

Because it is glass, obsidian is metastable and eventually 'devitrifies' into crystalline quartz; thus, it is rarely found in rocks older than the Cretaceous period. It was one of the most highly traded materials in prehistoric times for making arrowheads and mirrors.

Field identification & locations

Identify by its glass-like texture, conchoidal (shell-like) fractures at the drill holes, and the unique internal 'glow' when rotated under a strong light source. Commonly found in Iceland, Mexico, and the Western United States (Oregon/California).

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