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Snowflake Obsidian is a naturally occurring volcanic glass characterized by its opaque black or dark gray groundmass punctuated by white to grayish-white, radially structured mineral inclusions that resemble snowflakes. These inclusions are spherulites composed primarily of cristobalite, a high-temperature polymorph of silica. The contrast between the dark glass and the light spherulites gives the rock its distinctive appearance. It is a relatively hard and brittle material, exhibiting a characteristic conchoidal fracture.
How to Identify
- Color
- Black to dark gray groundmass with white to grayish-white 'snowflake' patterns.
- Luster
- Vitreous (glassy) on fractured surfaces.
- Texture
- Smooth, glassy groundmass with slightly rougher, granular spherulites.
- Crystal Form
- Amorphous (glassy) groundmass; spherulitic aggregates of microcrystalline cristobalite.
- Cleavage
- None, exhibits conchoidal fracture.
- Geological Environment
- Associated with rhyolitic volcanic flows, domes, and ignimbrite deposits, typically in areas of active or geologically recent volcanism.
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) with spherulitic aggregates of tetragonal cristobalite
- Color: Black to dark gray with white to grayish-white inclusions
- Luster: Vitreous
- Transparency: Opaque to translucent on thin edges
- Fracture: Conchoidal
- Cleavage: None
- Composition: Primarily SiO2 (70-75%), with minor amounts of Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, and H2O. The white 'snowflakes' are cristobalite (SiO2).
Quick Check
- Color: Black with white 'snowflakes'
- Luster: Vitreous (glassy)
- Streak: White (due to the fine powder of the cristobalite spherulites, though obsidian itself has no streak)
Physical Characteristics
- Crystal Habit: Amorphous (glassy) groundmass; spherulitic aggregates of microcrystalline cristobalite.
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous
Formation
Snowflake Obsidian forms from the rapid cooling of viscous, silica-rich (rhyolitic) lava. The rapid quenching prevents the formation of large, ordered mineral crystals, resulting in a glassy texture. The distinctive 'snowflake' patterns are spherulites of cristobalite, a high-temperature polymorph of silica (SiO2), which crystallize radially from a central point within the obsidian glass. These spherulites form during the later stages of cooling or through devitrification (crystallization of glass) over geological time, often due to the presence of water or other volatiles acting as catalysts.
Usage
Primarily used as an ornamental stone, for carving, cabochons, beads, and decorative objects. Historically, like other obsidians, it may have been used for tools, arrowheads, and cutting implements due to its conchoidal fracture, though its aesthetic appeal makes it more valued for decorative purposes today.
Age Distribution
Typically Cenozoic, but can form whenever volcanic activity produces rhyolitic lavas.
Where to Find
United States
Oregon, Utah, Arizona, California, and Nevada are significant sources, particularly in areas with extensive rhyolitic volcanism.
Mexico
Various volcanic regions.
Iceland
Volcanic areas.
Italy
Lipari and other volcanic islands.
Greece
Milos island.
Japan
Volcanic regions.
Argentina
Patagonia region.
Finding Tips
Look in Volcanic Regions
Snowflake Obsidian is found in areas of recent or geologically young rhyolitic volcanic activity. Search near lava flows, volcanic domes, and ashfall deposits.
Examine Outcrops and Stream Beds
Look for dark, glassy fragments in outcrops or eroded pieces in stream beds and alluvial fans downstream from volcanic sources.
Check for Conchoidal Fracture
Obsidian, including Snowflake Obsidian, will break with a characteristic conchoidal (shell-like) fracture, producing sharp edges.
Identify White Inclusions
The key identifying feature is the presence of white to grayish-white 'snowflake' patterns (cristobalite spherulites) within the dark glass.
Safety Precautions
Obsidian produces extremely sharp edges when fractured. Handle with care to avoid cuts. Wear appropriate gloves and eye protection when breaking or collecting samples. While the silica in cristobalite is crystalline, the primary hazard from obsidian is physical injury from sharp edges, not typically respirable crystalline silica unless finely powdered, which is not a common exposure for collectors.
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 (Obsidian) with Silicate Mineral Inclusions (Cristobalite)
- Group
- Igneous Rock
- Crystal System
- Amorphous (for the glass); Tetragonal (for cristobalite)
- Chemical Formula
- SiO2 (amorphous) + SiO2 (cristobalite)
- Composition
- Silica-rich volcanic glass with crystalline silica (cristobalite) spherulites.
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