Obsidian
Volcanic Glass (Amorphous Silica-rich Rock) · Semi-Precious Gemstone / Igneous Rock / Decorative Stone
Crystal System
Amorphous (Non-crystalline structure); Habit: Massive, exhibiting prominent conchoidal (shell-like) fracture patterns.
Mineral Group
Silicates; specifically a volcanic glass rather than a true mineral species due to lack of crystalline structure.
Chemical Formula
SiO2 (approx. 70–75%) with MgO, Fe3O4, and Al2O3
Formation Process
Igneous/Volcanic; forms when high-silica (rhyolitic) lava cools rapidly at the surface, preventing crystal growth.
About This Crystal
Jet-black, opaque to slightly translucent at thin edges. Features a high vitreous (glassy) luster and smooth, curved fracture surfaces.
Physical Characteristics
Color: Black; Streak: White; Cleavage: None; Fracture: Conchoidal; Specific Gravity: 2.35–2.60; Tenacity: Brittle; Magnetism: Occasionally weak due to magnetite micro-inclusions.
Optical Properties
Isotropic (singly refractive); Refractive Index: 1.45–1.55; Lacks pleochroism and birefringence; can show sheen if tiny bubbles or inclusions are present.
Hardness & Durability
Origin Region
Common in volcanic regions: Mexico, United States (Oregon, California), Iceland, Japan, and Italy (Lipari).
Hardness & Durability
Mohs Hardness: 5.0–6.0; Brittle toughness; sensitive to sudden temperature changes and hard impacts due to easy chipping.
Care & Maintenance
Wash with warm soapy water; avoid ultrasonic or steam cleaners. Store separately from harder stones like quartz to prevent scratching.
Rarity & Value
Common; Value is generally low but increases for varieties like Rainbow, Fire, or Snowflake Obsidian; rarely treated but can be imitated by slag glass.
Special Characteristics
Displays prominent conchoidal fracture that produces extremely sharp edges, used historically for surgical and hunting tools.
Lore & History
Used by ancient Mesoamericans for mirrors and weapons (macuahuitl); traditionally associated with protection, grounding, and revealing hidden truths.