Obsidian
Volcanic Glass (Amorphous Silica) · Semi-Precious Gemstone / Volcanic Rock
Crystal System
Amorphous (Non-crystalline); Typically massive or cobble habit with no internal geometric structure.
Mineral Group
Silicates (Non-mineraloid Volcanic Glass)
Chemical Formula
SiO2 (70–75%) + Al2O3, Na2O, K2O, CaO, MgO, Fe3O4
Formation Process
Igneous: Forms when high-silica felsic lava cools so rapidly (quenching) that silicate crystals do not have time to grow, resulting in a glass.
About This Crystal
Jet-black, opaque to translucent at thin edges, featuring a brilliant vitreous (glassy) luster. The specimen shows shell-like curved surfaces indicating conchoidal fracture.
Physical Characteristics
Color: Black; Streak: White; Cleavage: None; Fracture: Conchoidal; Specific Gravity: 2.35–2.60; Tenacity: Brittle; Fluorescence: Generally inert.
Optical Properties
Isotropic (Singly Refractive) due to amorphous nature; Refractive Index: 1.450–1.550; No birefringence or pleochroism.
Hardness & Durability
Origin Region
Notable sources include Mexico, United States (Oregon, California), Iceland, Japan, and Italy (Lipari). Found in geologically recent volcanic areas.
Hardness & Durability
Mohs Hardness: 5–6; Brittle toughness; Sensitive to heat and impact; Wearable but prone to scratching compared to quartz or beryl.
Care & Maintenance
Wash with warm soapy water and a soft cloth; avoid ultrasonic cleaners or steam; store separately to prevent scratching; avoid extreme temperature shocks.
Rarity & Value
Common; Value is generally low for raw specimens, though specific varieties like Rainbow or Fire Obsidian command higher prices. Usually untreated.
Special Characteristics
Displays perfect conchoidal fracture which allows it to be broken into edges thinner and sharper than high-quality steel scalpels.
Lore & History
Historically used for arrowheads, blades, and ritual mirrors in Mesoamerica. Metaphysical lore associates it with protection, grounding, and the Root Chakra.