Obsidian
Volcanic Glass (Amorphous Silica) · Semi-Precious Gemstone / Decorative Stone
Crystal System
Amorphous (no crystal structure); occurs in massive form
Mineral Group
Silicates (Mineraloid)
Chemical Formula
70–75% SiO2, plus MgO, Fe3O4
Formation Process
Igneous: Forms from the rapid cooling of silica-rich rhyolitic lava which prevents crystal growth
About This Crystal
Jet black to dark grey, opaque to translucent at thin edges, vitreous (glassy) luster with a smooth, carved surface as seen in the specimen
Physical Characteristics
Black color, white streak, no cleavage, conchoidal (shell-like) fracture, specific gravity 2.35–2.60, non-fluorescent
Optical Properties
Isotropic (singly refractive), refractive index 1.45–1.55, typically lacks birefringence except under internal strain
Hardness & Durability
Origin Region
Mexico, USA (Oregon/California), Iceland, and Japan; found in volcanic regions
Hardness & Durability
Mohs hardness 5.0–6.0; brittle tenacity; susceptible to scratching and chipping due to glass-like nature
Care & Maintenance
Clean with warm soapy water and a soft cloth; avoid ultrasonic cleaners, steam, and contact with harder gems like quartz or sapphire
Rarity & Value
Common; value determined by carving quality and variety; generally inexpensive but priced higher for specific varieties like Rainbow or Fire Obsidian
Special Characteristics
Exhibits conchoidal fracture which creates extremely sharp edges; thin edges may appear translucent or smoky brown when backlit
Lore & History
Historically used by ancient cultures (Aztecs, Mayans) for mirrors and sacrificial blades; traditionally believed to be a stone of protection and grounding