Rainbow Obsidian
Volcanic Glass (Amorphous Silica) with oriented nanoparticle inclusions · Semi-Precious Gemstone / Mineral Specimen
Crystal System
Amorphous (Non-crystalline structure); commonly found in massive habits resulting from rapid cooling of lava
Mineral Group
Silicates (specifically a mineraloid rather than a mineral due to its glass-like nature)
Chemical Formula
SiO2 (70-75%) with Al2O3, Na2O, K2O, and trace Fe-magnetite nanoinclusions
Formation Process
Igneous: Forms when high-silica (rhyolitic) lava cools rapidly at the edges of a lava flow, preventing crystal growth. The 'rainbow' effect is caused by nanofeatures of magnetite or pyroxene.
About This Crystal
A dark, translucent to opaque volcanic glass. While it appears black or deep charcoal under normal lighting, direct light reveals concentric bands of spectral colors (purple, green, blue, gold) caused by light diffraction on microscopic inclusions.
Physical Characteristics
Color: Black to dark brown with iridescent sheen; Streak: White; Luster: Vitreous (glassy); Fracture: Conchoidal (shell-like); Specific Gravity: 2.35 - 2.60; Tenacity: Brittle; Fluorescence: None.
Optical Properties
Isotropic (Singly Refractive); Refractive Index: 1.48 - 1.51; Displays iridescence/schiller effect via thin-film interference or diffraction on microscopic inclusions; No pleochroism.
Hardness & Durability
Origin Region
Primarily Jalisco and Nayarit, Mexico; also found in the Western United States (Oregon and California)
Hardness & Durability
Mohs Hardness: 5.0 - 5.5; Low toughness due to conchoidal fracture and brittleness; susceptible to scratching and chipping; stable under most light and chemical conditions.
Care & Maintenance
Clean with warm soapy water and a soft cloth; avoid ultrasonic or steam cleaners as they may cause fracturing; store separately from harder stones like quartz or corundum to prevent surface scratching.
Rarity & Value
Common; Value is determined by the brightness and variety of colors in the 'eye' or layers; polished tumbled stones are affordable, while high-quality carvings or cabochons are valued more by collectors.
Special Characteristics
Displays 'Rainbow Iridescence' or 'Schiller' effect; when cut as a sphere or cabochon, the internal pressure bands often form an 'eye' shape or concentric patterns of changing colors.
Lore & History
Historically used by ancient Mesoamerican cultures for mirrors, weapons (macuahuitl), and ritual objects. Metaphysically associated with the Root Chakra and believed to provide protection and help in healing emotional wounds by absorbing negativity.