Obsidian
Volcanic Glass (Amorphous Silica) · Extrusive Igneous Rock / Semi-Precious Gemstone
Crystal System
Amorphous (Non-crystalline); typical habit is massive, showing conchoidal fracture patterns.
Mineral Group
Silicates (specifically a mineraloid glass)
Chemical Formula
SiO2 (approx. 70-75%) plus MgO, Fe3O4
Formation Process
Igneous. It forms when felsic lava extruded from a volcano cools so rapidly that crystal growth is inhibited, resulting in a volcanic glass.
About This Crystal
A dark, glassy mineraloid with a predominant jet-black to dark green color. It exhibits a highly vitreous luster and is typically opaque to translucent on thin edges. The surface often shows smooth, curved shell-like (conchoidal) breakage lines.
Physical Characteristics
Color: Black; Streak: White; Cleavage: None; Fracture: Conchoidal; Specific Gravity: 2.35 - 2.60; Tenacity: Brittle; Fluorescence: Generally inert.
Optical Properties
Refractive Index: 1.450 - 1.550; Single Refraction (Isotropic); Pleochroism: None; Luster: Vitreous; Phenomenon: May show gold or silver sheen (sheen obsidian) or iridescence.
Hardness & Durability
Origin Region
Western USA (Oregon, California), Mexico, Iceland, and Japan; found in areas of geologically recent volcanic activity.
Hardness & Durability
Mohs Hardness: 5.0 - 5.5. Brittle toughness; sensitive to pressure and heat. Moderate wearability due to low hardness compared to quartz.
Care & Maintenance
Clean with warm soapy water and a soft cloth; avoid ultrasonic or steam cleaners. Store separately from harder stones like diamonds or sapphires to prevent scratching.
Rarity & Value
Common. Value factors include color patterns (sheen, snowflake, rainbow) and size. Generally inexpensive, ranging from $1 to $20 for small raw or tumbled specimens.
Special Characteristics
Distinctive conchoidal fracture allowing for extremely sharp edges; some varieties exhibit sheen caused by gas bubbles or snowflake-like inclusions of cristobalite.
Lore & History
Historically used for making extremely sharp blades, arrowheads, and mirrors in pre-Columbian Mesoamerica. In traditional lore, it is considered a protective stone used for grounding and shielding against negativity.