Igneous

Rainbow Obsidian

Volcanic glass (SiO2 - approximately 70% or more silicon dioxide)

Hardness: 5-5.5 (Mohs scale); Color: Black with colorful iridescence (purple, green, gold); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Hardness: 5-5
Color: Black with colorful iridescence (purple, green, gold)
Luster: Vitreous/Glassy

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. The 'rainbow' effect is caused by microscopic inclusions of magnetite or nanobubbles of gas trapped during flow. Geological age varies by site, typically Cenozoic Era.

Uses & applications

Used primarily in jewelry (beads, cabochons), ornamental carvings, and for spiritual/metaphysical collecting. Historically used for extremely sharp cutting tools and scalpels.

Geological facts

The rainbow sheen is actually an optical phenomenon caused by light diffraction on layers of mineral inclusions rather than pigment. It is one of the sharpest naturally occurring materials; its edges can be thinner than a molecule of steel.

Field identification & locations

Identify in the field by its glassy texture and conchoidal 'shell-like' fractures. While it looks solid black in shade, the rainbow sheen is revealed under direct, bright light. Common locations include Mexico (Jalisco), Northern California (Glass Mountain), and Oregon.

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