Obsidian
Volcanic Glass (Amorphous Silicon Dioxide) · Mineraloid / Decorative Stone
Crystal System
Amorphous (Non-crystalline); Massive habit with no regular crystal structure
Mineral Group
Silicates (Mineraloid)
Chemical Formula
SiO2 (70–75%) plus MgO, Fe3O4
Formation Process
Igneous; forms when felsic lava extrudes from a volcano and cools so rapidly that crystals do not have time to grow, resulting in a natural glass.
About This Crystal
The specimen shows a dark, smoky to greenish-black body color with a vitreous to greasy luster. It displays a characteristic conchoidal fracture (curved, shell-like ridges) visible at the top breakage point. It is translucent on thin edges but appears opaque in thick masses.
Physical Characteristics
Color: Black, dark green, or gray; Streak: White; Cleavage: None; Fracture: Conchoidal; Specific Gravity: 2.35–2.6; Tenacity: Brittle; Fluorescence: Generally none.
Optical Properties
Isotropic (singly refractive) due to amorphous nature; Refractive Index: 1.450–1.550; No pleochroism or birefringence.
Hardness & Durability
Origin Region
Geographically diverse; common in Iceland, Mexico, USA (Oregon/California), and Japan. Found in volcanic rhyolitic flows.
Hardness & Durability
Mohs Hardness: 5.0 to 6.0; Brittle toughness; sensitive to sudden temperature changes and physical impact; susceptible to scratching compared to quartz.
Care & Maintenance
Clean with warm soapy water and a soft cloth; avoid ultrasonic or steam cleaners; store separately from harder gems like sapphire or quartz to prevent scuffing.
Rarity & Value
Common; Value is generally low for massive specimens, though specific varieties like Rainbow or Fire Obsidian command higher prices. Usually untreated.
Special Characteristics
Displays prominent conchoidal fracturing; sharp edges when broken; some varieties exhibit mahogany-colored streaks or microscopic inclusions that create sheen.
Lore & History
Historically used by ancient cultures (Aztecs, Mayans) for mirrors, sacrificial daggers, and arrowheads due to its razor-sharp edges. Spiritually associated with protection, grounding, and the root chakra.