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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 inclusions that resemble snowflakes. These inclusions are spherulites composed of the mineral cristobalite, a high-temperature polymorph of silica (SiO2). The formation of these spherulites indicates a partial devitrification of the obsidian glass, a process where the amorphous glass begins to crystallize. The contrast between the glassy, dark matrix and the opaque, lighter spherulites gives Snowflake Obsidian its distinctive and attractive appearance.
How to Identify
- Color
- Black to dark gray groundmass with white to grayish-white 'snowflake' patterns.
- Luster
- Vitreous (glassy) on fresh surfaces, duller on weathered surfaces.
- Texture
- Smooth, conchoidal fracture. The 'snowflakes' are slightly rougher to the touch than the glassy matrix.
- Crystal Form
- Amorphous (glassy) matrix with spherulitic (radial, spherical aggregates) cristobalite inclusions.
- Cleavage
- None (due to its amorphous nature).
- Geological Environment
- Found in areas of recent felsic volcanic activity, typically associated with rhyolitic lava flows or domes.
Key Facts
- Hardness: 5-5.5 on the Mohs scale (for the glass matrix); cristobalite is slightly harder at 6-7.
- Specific Gravity: 2.35-2.50
- Crystal System: Amorphous (glassy) for the obsidian matrix; 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 'snowflakes' are almost pure SiO2 (cristobalite).
Quick Check
- Color: Black with white 'snowflakes'
- Luster: Vitreous (glassy)
- Streak: White (due to its amorphous nature and high silica content, it does not produce a colored streak)
Physical Characteristics
- Crystal Habit: Amorphous (glassy) matrix; cristobalite forms spherulitic aggregates.
- Cleavage Type: Absent.
- Fracture Type: Conchoidal, producing very sharp edges.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy).
Formation
Snowflake Obsidian forms from the rapid cooling of felsic lava (rich in silica), preventing the formation of large crystals. The characteristic 'snowflakes' are spherulitic growths of cristobalite, a high-temperature polymorph of silica, which crystallize within the obsidian glass as it devitrifies (crystallizes) over time or during slower cooling phases within the obsidian flow.
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 and weapons by indigenous cultures, though its aesthetic qualities make it more valued for decorative purposes today.
Age Distribution
Typically Cenozoic, associated with recent volcanic activity (tens of millions of years to present).
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
Known for its diverse obsidian occurrences.
Italy
Specifically the island of Lipari.
Greece
The island of Milos.
Finding Tips
Geological Context
Look in areas with recent (Cenozoic) felsic volcanic activity, particularly near rhyolite flows, domes, or volcanic ash deposits. Obsidian often occurs as flow bands, nodules, or larger masses within these settings.
Surface Features
Obsidian weathers slowly, but exposed surfaces may show a duller luster or hydration rinds. Freshly broken surfaces will exhibit the characteristic glassy luster and conchoidal fracture.
Associated Minerals
May be found alongside other volcanic rocks like rhyolite, pumice, and perlite. Other silica polymorphs or zeolites might be present in the surrounding rock.
Safety Precautions
Obsidian breaks with extremely sharp edges, similar to glass. Always wear sturdy gloves and eye protection when handling or collecting. Be aware of unstable terrain in volcanic areas.
Similar Rocks
Black Obsidian
Obsidian
Also known as: Volcanic Glass
Mahogany Obsidian
Obsidian with Iron Oxide Inclusions
Also known as: Red Obsidian
Apache Tears
Obsidian Nodules
Also known as: Marekanite
Scientific Classification
- Mineral Class
- Volcanic Glass (Obsidian) with Silica Polymorph Inclusions
- Group
- Silicate (though technically a glass, not a mineral in the strict sense for the matrix)
- Crystal System
- Amorphous (for obsidian); Tetragonal or Orthorhombic (for cristobalite inclusions)
- Chemical Formula
- SiO2 (amorphous) + SiO2 (cristobalite)
- Composition
- Felsic volcanic glass, predominantly silica (70-75% SiO2), with spherulitic inclusions of cristobalite.
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