Obsidian
Volcanic Glass var. Obsidian · Collector Mineral / Decorative Stone
Crystal System
Amorphous (Non-crystalline); habit is typically massive or pebble-like with smooth, curved surfaces.
Mineral Group
Silicates (specifically a mineraloid)
Chemical Formula
SiO2 (approx. 70-75%) with MgO, Fe3O4
Formation Process
Igneous (Extrusive); forms when high-silica lava cools extremely rapidly, preventing internal crystal growth.
About This Crystal
Deep black, opaque to translucent at thin edges, featuring a vitreous (glassy) luster and smooth, undulating surfaces typical of water-worn or weathered volcanic glass.
Physical Characteristics
Color: Jet black; Streak: White; Cleavage: None; Fracture: Conchoidal; Specific Gravity: 2.35–2.6; Fluorescence: Generally none.
Optical Properties
Isotropic (singly refractive); Refractive Index: 1.48–1.51; No pleochroism or birefringence due to lack of crystal structure.
Hardness & Durability
Origin Region
Volcanic regions worldwide (Mexico, USA, Iceland, Greece); forms in areas with geologically recent extrusive activity.
Hardness & Durability
Mohs Hardness: 5.0–5.5; Toughness: Low (brittle and prone to chipping/fracturing); Sensitivity: Heat and physical impact.
Care & Maintenance
Clean with warm soapy water and a soft cloth; avoid ultrasonic cleaners and harsh chemicals; store separately to prevent scratching from harder minerals like quartz.
Rarity & Value
Common; value determined by size and presence of unique inclusions (like sheen or flecks); very affordable as a raw specimen or tumbled stone.
Special Characteristics
Conchoidal fracture produces razor-sharp edges; this specimen appears to have surface weathering or 'pitting' from environmental exposure.
Lore & History
Historically used for making surgical blades, arrowheads, and mirrors in ancient Mesoamerica; traditionally associated with grounding, protection, and the root chakra.