Obsidian
Volcanic Glass (Amorphous Silica) · Semi-Precious Gemstone / Mineraloid Specimen
Crystal System
Amorphous (Non-crystalline); lacks an internal atomic structure. Habit is typically massive or as rounded nodules (Apache tears).
Mineral Group
Mineraloid (Silicate-rich volcanic glass)
Chemical Formula
SiO2 (70–75%) with inclusions of MgO, Fe3O4
Formation Process
Igneous; formed by the extremely rapid cooling of felsic (silica-rich) lava, which prevents crystal growth.
About This Crystal
Opaque to translucent deep black glass with a vitreous to sub-vitreous luster. Features prominent vesicles (gas bubbles) as seen in the specimen, which suggest a surface flow origin.
Physical Characteristics
Color: Jet black; Streak: White; Cleavage: None; Fracture: Conchoidal (shell-like); Specific Gravity: 2.35–2.6; Fluorescence: Generally inert.
Optical Properties
Isotropic (singly refractive); Refractive Index: 1.450–1.550; Lacks pleochroism and birefringence due to amorphous nature.
Hardness & Durability
Origin Region
Common in volcanic regions such as Iceland, Mexico, USA (Oregon/California), and the Aeolian Islands of Italy.
Hardness & Durability
Mohs Hardness: 5–5.5; Brittle tenacity; Fair durability for jewelry, though prone to scratching and chipping.
Care & Maintenance
Clean with warm soapy water; avoid ultrasonic cleaners and steam cleaning. Store separately from harder stones like quartz or topaz to prevent scratches.
Rarity & Value
Common; Value is generally low ($2 - $100 depending on size/carving). Value increases for varieties like Rainbow, Fire, or Snowflake obsidian.
Special Characteristics
Displays perfect conchoidal fracture allowing for razor-sharp edges. This specimen specifically shows 'Vesicular' texture from trapped volcanic gases.
Lore & History
Historically used by ancient cultures (Aztecs, Mayans) for mirrors, knives, and sacrificial daggers. Metaphysically regarded as a protective 'vacuum' stone for negativity.