Igneous
Rainbow Obsidian
Volcanic Glass (SiO2 + impurities)
Hardness: 5-5.5 (Mohs); Color: Black with iridescent multicolored sheen; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black with iridescent multicolored sheen
- Luster
- Vitreous/Glassy
Identified More igneous →
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Black with iridescent multicolored sheen; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.35-2.6
Formation & geological history
Formed from the rapid cooling of silica-rich rhyolitic lava. The 'rainbow' effect is caused by tiny inclusions of magnetite or oriented nanoparticles/bubbles trapped during the cooling process.
Uses & applications
Primarily used for lapidary work, jewelry (cabochons), pocket stones, and decorative carvings. Historically used for sharp cutting tools and arrowheads.
Geological facts
Obsidian is technically a supercooled liquid rather than a true mineral because it lacks a crystalline structure. The rainbow effect is most visible when the stone is wet or polished and held under a bright direct light source.
Field identification & locations
Identify by its glassy texture, shell-like (conchoidal) fracture pattern, and the subtle, concentric 'eye' layers of color. Commonly found in volcanic regions such as Oregon, Mexico, and Iceland.
