Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Snowflake Obsidian is a naturally occurring volcanic glass characterized by its black or dark gray base color, punctuated by distinctive white or grayish 'snowflake' patterns. These patterns are spherulitic growths of cristobalite, a silica mineral, that form within the amorphous obsidian matrix. The contrast between the glassy, dark obsidian and the opaque, lighter cristobalite inclusions gives it its unique aesthetic appeal. It is a relatively hard and brittle material, exhibiting a conchoidal fracture.
How to Identify
- Color
- Black to dark gray with white to light gray 'snowflake' patterns.
- Luster
- Vitreous (glassy) for the obsidian matrix; dull to sub-vitreous for the cristobalite inclusions.
- Texture
- Smooth, glassy matrix with slightly rougher, granular inclusions. Amorphous overall.
- Crystal Form
- Amorphous (glassy) matrix; the cristobalite forms radial, spherical aggregates (spherulites) within the glass.
- Cleavage
- None (exhibits conchoidal fracture).
- Geological Environment
- Found in areas of recent or geologically young felsic volcanic activity, typically associated with rhyolitic lava flows or domes where rapid cooling occurred, followed by partial devitrification.
Key Facts
- Hardness: 5-5.5 on Mohs scale (for the obsidian matrix); cristobalite is slightly harder, around 6-7.
- Specific Gravity: 2.35-2.60 g/cm³
- Crystal System: Amorphous (obsidian matrix); Tetragonal (cristobalite inclusions, though often cryptocrystalline within spherulites)
- Color: Black to dark gray with white to light gray inclusions
- Luster: Vitreous (glassy)
- Transparency: Opaque to translucent on thin edges
- Fracture: Conchoidal
- Cleavage: None
- Composition: Primarily SiO2 (silica glass) with minor amounts of Al2O3, Fe2O3, Na2O, K2O, and H2O. The white inclusions are crystalline SiO2 (cristobalite).
Quick Check
- Color: Black with white spots
- Luster: Glassy
- Streak: White (due to the cristobalite, though the obsidian itself has no streak)
Physical Characteristics
- Crystal Habit: Amorphous (glassy) matrix with spherulitic (radial, spherical) aggregates of cristobalite.
- Cleavage Type: None
- Fracture Type: Conchoidal, producing very sharp edges.
- Tenacity: Brittle
- Luster Type: Vitreous (glassy) for the obsidian; dull to sub-vitreous for the cristobalite.
Formation
Snowflake Obsidian forms from the rapid cooling of felsic lava (high silica content, typically rhyolitic) without significant crystal growth. The 'snowflakes' are spherulitic inclusions of cristobalite, a high-temperature polymorph of silica (SiO2), which crystallize radially from a central point within the obsidian glass as it devitrifies (crystallizes) over time or due to specific cooling conditions. This devitrification process is often initiated by the presence of water or other volatiles, or by slight variations in temperature and pressure during the cooling phase. The cristobalite spherulites are typically white or gray and contrast sharply with the dark, glassy matrix.
Usage
Primarily used as an ornamental stone, for carving, cabochons, beads, and other lapidary applications. Historically, like other obsidians, it could have been used for tools, but its aesthetic value makes it more suitable for decorative purposes today. It is also collected by mineral and rock enthusiasts.
Age Distribution
Typically Cenozoic, but can form in any geologically recent volcanic activity where conditions are met.
Where to Find
United States
Oregon, Utah, Arizona, California, and Nevada are prominent sources, particularly in areas with extensive rhyolitic volcanism.
Mexico
Various volcanic regions in Mexico yield Snowflake Obsidian.
Iceland
Known for its diverse volcanic rocks, including obsidian varieties.
Italy
Specifically the island of Lipari, known for its obsidian deposits.
Greece
The island of Milos is a historical source of obsidian.
Finding Tips
Geological Context
Look for Snowflake Obsidian in areas with recent (Cenozoic) felsic volcanic activity, particularly near rhyolitic lava flows, domes, or volcanic necks. It often occurs in the outer margins of these flows where cooling was rapid.
Associated Minerals
It may be found alongside other forms of obsidian, pumice, and sometimes perlite. The surrounding rock will typically be volcanic in origin.
Surface Identification
The distinctive white 'snowflakes' against the dark glassy background are usually visible on weathered or broken surfaces. Look for shiny, dark fragments with these characteristic patterns.
Safety Precautions
Obsidian, including Snowflake Obsidian, breaks with extremely sharp edges. Always wear appropriate gloves and eye protection when handling raw specimens or breaking them. The dust generated from cutting or grinding obsidian (which is primarily silica) can be a respiratory hazard; use proper ventilation and dust masks (e.g., N95 or P100 respirators) to prevent silicosis. While the cristobalite is embedded, prolonged exposure to airborne cristobalite dust is a known carcinogen.
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 minerals)
- Group
- Silicate (specifically, a natural glass with silica polymorph inclusions)
- Crystal System
- Amorphous (for the glass); Tetragonal (for cristobalite)
- Chemical Formula
- SiO2 (amorphous) + SiO2 (cristobalite)
- Composition
- Predominantly silicon dioxide (SiO2) with varying trace elements and water content. The 'snowflakes' are microcrystalline cristobalite.
Explore Snowflake Obsidian
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.