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Snowflake Obsidian is a naturally occurring volcanic glass characterized by its black or dark gray groundmass punctuated by white to grayish-white, radially structured spherulites of cristobalite, which resemble snowflakes. The contrast between the dark glass and the light spherulites creates its unique and appealing aesthetic. It is an amorphous material, meaning it lacks a defined crystal structure, except for the embedded cristobalite inclusions.
How to Identify
- Color
- Black to dark gray groundmass with white to grayish-white 'snowflake' patterns (cristobalite spherulites).
- Luster
- Vitreous (glassy) to sub-vitreous.
- Texture
- Smooth, conchoidal fracture, with embedded granular or radial spherulitic inclusions.
- Crystal Form
- Amorphous (glassy) groundmass; cristobalite inclusions are typically spherulitic (radial aggregates of microscopic crystals).
- Cleavage
- None (due to its amorphous nature).
- Geological Environment
- Formed in areas of explosive felsic volcanism, typically associated with rhyolitic lava flows, domes, and ignimbrite deposits. It occurs in volcanic regions where rapid cooling of silica-rich magma prevents crystallization, followed by subsequent devitrification to form cristobalite spherulites.
Key Facts
- Hardness: 5-5.5 on Mohs scale (for the glass), cristobalite is slightly harder at 6-7
- Specific Gravity: 2.35-2.60
- Crystal System: Amorphous (glass); Cristobalite is tetragonal (high-temperature form) or orthorhombic (low-temperature form)
- Color: Black to dark gray with white to grayish-white inclusions
- Luster: Vitreous (glassy)
- Transparency: Opaque to translucent in thin sections
- Fracture: Conchoidal (shell-like)
- Cleavage: None
- Composition: Primarily SiO2 (silica) with minor amounts of Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, and H2O. The white 'snowflakes' are composed of cristobalite (SiO2).
Quick Check
- Color: Black with white 'snowflakes'
- Luster: Glassy
- Streak: White (due to its hardness, it will scratch a streak plate rather than leave a streak)
Physical Characteristics
- Crystal Habit: Amorphous (glassy) groundmass; cristobalite inclusions are spherulitic (radial aggregates).
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous (glassy)
Formation
Snowflake Obsidian forms from the rapid cooling of felsic lava (rhyolitic composition) with a high silica content, preventing the formation of crystalline structures and resulting in a glassy texture. The distinctive 'snowflake' patterns are spherulitic inclusions of cristobalite, a high-temperature polymorph of silica (SiO2). These cristobalite spherulites crystallize from the glass during devitrification, a process where the amorphous glass begins to crystallize, often due to slow cooling after initial rapid quenching, or post-emplacement alteration. The spherulites grow radially from a central nucleation point.
Usage
Primarily used as an ornamental stone, for carving, cabochons, beads, and decorative objects. Historically, like other obsidians, it could have been used for tools, arrowheads, and cutting implements, though its aesthetic qualities make it more valued for decorative purposes today. It is also popular in lapidary arts and as a collector's specimen.
Age Distribution
Typically Cenozoic to recent, associated with active volcanism.
Where to Find
United States
Particularly in volcanic regions of the western U.S., such as Oregon (Glass Buttes), Utah, Arizona, and California.
Mexico
Known occurrences in various volcanic areas.
Iceland
Associated with its extensive volcanic activity.
Japan
Found in areas with recent volcanic history.
Argentina
Occurrences in the Andes volcanic belt.
Finding Tips
Volcanic Regions
Focus your search in areas with recent or geologically young felsic volcanic activity. Look for lava flows, volcanic domes, and areas where volcanic glass is known to occur.
Outcrops and Talus Slopes
Snowflake obsidian can be found in outcrops of obsidian flows or as loose fragments in talus slopes and stream beds downstream from volcanic sources. Look for dark, glassy rocks with distinct white patterns.
Safety Precautions
Obsidian, including Snowflake Obsidian, breaks with extremely sharp edges. Handle with care to avoid cuts. Wear sturdy gloves and eye protection when collecting or breaking specimens. When cutting or grinding, use appropriate respiratory protection (e.g., N95 mask) to avoid inhaling fine silica dust, which can cause silicosis.
Similar Rocks
Obsidian
Obsidian
Also known as: Volcanic Glass
Apache Tears
Obsidian Nodules
Also known as: Marekanite
Pumice
Pumice
Also known as: Volcanic Foam
Scientific Classification
- Mineral Class
- Volcanic Glass (not a true mineral due to amorphous structure, but composed of mineraloids and mineral inclusions)
- Group
- Silicate (specifically, a natural glass of felsic composition)
- Crystal System
- Amorphous (for the glass); Tetragonal or Orthorhombic (for cristobalite inclusions)
- Chemical Formula
- Predominantly SiO2 (amorphous) with embedded SiO2 (cristobalite)
- Composition
- Silica-rich volcanic glass (70-75% SiO2) with spherulitic inclusions of cristobalite (SiO2).
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