Rainbow Obsidian
Volcanic Glass (Amorphous Silica) · Semi-Precious Gemstone / Decorative Stone
Crystal System
Amorphous (Non-crystalline); Typically occurs as massive blocks or rounded nodules.
Mineral Group
Silicates (Non-mineraloid Volcanic Glass)
Chemical Formula
70–75% SiO2, plus MgO, Fe3O4 (Magnetite inclusions produce the iridescence)
Formation Process
Igneous: Forms when felsic lava extrudes from a volcano and cools so rapidly that crystal growth cannot occur.
About This Crystal
Deep black to dark brown opaque glass with a vitreous luster. When polished and viewed under bright light, it reveals concentric 'rainbow' bands of color (purple, green, gold).
Physical Characteristics
Color: Black; Streak: White; Cleavage: None; Fracture: Conchoidal (shell-like); Specific Gravity: 2.35–2.60; Luster: Vitreous; Diaphaneity: Opaque to Translucent on thin edges.
Optical Properties
Isotropic (Singly refractive); RI: 1.48–1.51; Iridescence caused by light interference on layers of microscopic nanoparticles or water bubbles.
Hardness & Durability
Origin Region
Notably Mexico (Jalisco), United States (Oregon, California), and Iceland.
Hardness & Durability
Mohs Hardness: 5.0–5.5; Toughness: Poor (brittle and prone to chipping/cracking); Stability: Sensitive to sudden temperature changes.
Care & Maintenance
Clean with warm soapy water and a soft cloth; avoids ultrasonic cleaners or steam. Store away from harder stones like quartz or topaz to prevent scratching.
Rarity & Value
Common/Affordable; Value is determined by the vibrancy and orientation of the rainbow colors. Often heat-treated or fake 'glass' is sold, but natural obsidian is plentiful.
Special Characteristics
Showcases a 'rainbow' effect or sheen when oriented correctly to a light source. Exhibits perfect conchoidal fracture used historically for sharp tools.
Lore & History
Historically used by Mesoamerican cultures for mirrors and sacrificial blades. Metaphysically associated with the Root Chakra, believed to provide psychic protection and emotional healing.