Obsidian
Volcanic Glass (Amorphous Silica) · Minor Gemstone / Igneous Rock
Crystal System
Amorphous (Non-crystalline); lacks an organized internal lattice. Atomic structure is disordered due to rapid cooling.
Mineral Group
Silicates (Non-mineraloid Volcanic Glass)
Chemical Formula
SiO2 (approx. 70-75%) + MgO, Fe3O4
Formation Process
Igneous (Extrusive). It forms from the rapid cooling of felsic lava, high in silica, which creates a high viscosity that prevents crystal growth before solidification.
About This Crystal
A jet black to dark grey volcanic glass with a smooth, uniform texture in pure forms. It exhibits a highly glassy (vitreous) luster and produces sharp edges. This specific specimen shows complex surface textures and flow banding mixed with lithic fragments.
Physical Characteristics
Color: Black; Streak: White; Cleavage: None; Fracture: Conchoidal (shell-like); Specific Gravity: 2.35–2.6; Tenacity: Brittle; Luster: Vitreous to sub-metallic in sections.
Optical Properties
Refractive Index: 1.45–1.55 (Isotropic); Singly refractive; typically opaque to translucent on thin edges; no pleochroism due to amorphous nature.
Hardness & Durability
Origin Region
Volcanic regions worldwide including Mexico, USA (Oregon, California), Iceland, and Italy (Lipari). Found in areas with geologically recent extrusive volcanic activity.
Hardness & Durability
Mohs Hardness: 5.0–6.0; Brittle with poor toughness; prone to chipping and scratching. Similar in hardness to a glass plate or a sharp knife.
Care & Maintenance
Clean with warm soapy water and a soft cloth. Avoid ultrasonic cleaners and high heat. Store separately to prevent 'flea bites' or scratching from harder stones.
Rarity & Value
Relatively common in volcanic zones. Value is generally low for raw specimens but increases for varieties like Fire Obsidian or Rainbow Obsidian. Unlikely to be treated or synthesized.
Special Characteristics
Displays conchoidal fracture which allows it to break into razor-sharp edges. May contain gas bubbles, 'snowflakes' of cristobalite, or iridescent 'sheen' caused by nanoparticle inclusions.
Lore & History
Historically used by pre-Columbian Mesoamericans for sacrificial daggers, arrowheads, and mirrors. Traditionally associated with protection, grounding, and revealing hidden truths in folklore.