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
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Physical properties
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
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.
