Obsidian
Volcanic Glass (felsic) · Semi-Precious Gemstone / Mineral Specimen
Crystal System
Amorphous (Non-crystalline); Typically massve habit with conchoidal fracturing
Mineral Group
Extrusive Igneous Rock / Mineraloid (Silicate-rich glass)
Chemical Formula
SiO2 (approx. 70–75%) with MgO, Fe3O4
Formation Process
Igneous: Extrusive volcanic formation where high-silica lava cools so rapidly that crystal growth is inhibited, resulting in a natural glass.
About This Crystal
A dark, jet-black to deep smoky brown vitreous material. It is opaque in mass but can be translucent on thin edges. It features a highly reflective, glassy luster and distinct smooth, curved surfaces from breakage.
Physical Characteristics
Color: Black; Streak: White; Cleavage: None; Fracture: Perfect conchoidal (shell-like); Specific Gravity: 2.35–2.6; Tenacity: Brittle; Fluorescence: Generally inert.
Optical Properties
Isotropic (singly refractive); Refractive Index: 1.45–1.55; Luster: Vitreous; No pleochroism or birefringence due to amorphous structure.
Hardness & Durability
Origin Region
Mexico (Pachuca), USA (Glass Buttes, Oregon), Iceland, and Japan; found in geologically recent volcanic areas
Hardness & Durability
Mohs Hardness: 5.0–6.0; Toughness: Low (very fragile and prone to chipping/shattering); Stability: Sensitive to sudden temperature changes.
Care & Maintenance
Clean with warm soapy water and a soft cloth; avoid ultrasonic cleaners or steam. Store separately to prevent scratching softer stones or being chipped by harder ones. Use caution as edges can be surgically sharp.
Rarity & Value
Common; Value is generally low for raw specimens, though specific varieties like Fire or Rainbow Obsidian command higher prices. No common treatments.
Special Characteristics
Displays perfect conchoidal fracture which allows for the creation of extremely sharp edges; may contain microscopic inclusions of magnetite or hematite causing sheen or 'rainbow' effects in certain varieties.
Lore & History
Historically used by ancient civilizations (Mayans, Aztecs) for making blades, mirrors, and arrowheads. Traditionally associated with protection, grounding, and the Root Chakra in metaphysical practices; believed to act as a 'mirror to the soul' and used for scrying.