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Snowflake Obsidian is a naturally occurring volcanic glass characterized by its black or dark gray base color, interspersed with distinctive white or gray, radially structured inclusions that resemble snowflakes. These 'snowflakes' are spherulites, which are crystalline growths of minerals like cristobalite or feldspar within the amorphous obsidian matrix. The contrast between the dark glass and the lighter spherulites gives this rock its unique and appealing appearance. It is a relatively common variety of obsidian.
How to Identify
- Color
- Black to dark gray with white to light gray 'snowflake' patterns (spherulites).
- Luster
- Vitreous (glassy).
- Texture
- Smooth, conchoidal fracture, amorphous matrix with visible crystalline spherulites.
- Crystal Form
- Amorphous (glassy) matrix with radial, spherical aggregates of microcrystalline minerals (spherulites).
- Cleavage
- None (due to its amorphous nature).
- Geological Environment
- Volcanic regions where felsic lava cools rapidly, typically in lava flows, domes, or margins of dikes and sills.
Key Facts
- Hardness: 5 to 5.5 on the Mohs scale (due to its glassy nature, it is relatively brittle)
- Specific Gravity: 2.35 to 2.60 g/cm³
- Crystal System: Amorphous (lacks a crystalline structure in its bulk, though spherulites are microcrystalline)
- Color: Black to dark gray with white to light gray spherulites
- Luster: Vitreous (glassy)
- Transparency: Opaque to translucent on thin edges
- Fracture: Conchoidal (sharp, curved, shell-like fractures)
- Cleavage: None
- Composition: Primarily silicon dioxide (SiO₂) with minor amounts of aluminum, sodium, potassium, iron, and other elements. The spherulites are typically cristobalite (a high-temperature polymorph of SiO₂) or feldspar minerals.
Quick Check
- Color: Black with white 'snowflakes'
- Luster: Glassy
- Streak: White (due to its amorphous nature and high silica content, it does not produce a colored streak)
Physical Characteristics
- Crystal Habit: Amorphous mass with embedded radial spherulites.
- Cleavage Type: Absent.
- Fracture Type: Conchoidal, producing extremely sharp edges.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy).
Formation
Snowflake Obsidian forms from the rapid cooling of felsic lava (high silica content), preventing crystal growth and resulting in a glassy texture. The distinctive 'snowflake' patterns are spherulites, which are radial aggregates of microcrystalline feldspar (typically cristobalite or feldspar minerals like anorthoclase) that crystallize within the obsidian glass during devitrification. This devitrification can occur during the initial cooling phase or later through hydrothermal alteration or weathering processes. The spherulites grow outwards from a central nucleation point, forming spherical structures that appear white or gray against the dark obsidian matrix.
Usage
Primarily used as an ornamental stone, for lapidary work (cabochons, beads, carvings), and in jewelry. 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. It is also collected by mineral and rock enthusiasts.
Age Distribution
Typically Cenozoic, but can be found in any geological period with recent volcanic activity.
Where to Find
United States
Oregon, Utah, Arizona, California, and Nevada are significant sources, particularly in areas with recent rhyolitic volcanism.
Mexico
Various volcanic regions throughout Mexico yield Snowflake Obsidian.
Iceland
Known for its diverse volcanic rocks, including obsidian varieties.
Armenia
Historically a source of obsidian, including varieties with spherulitic textures.
Greece
The island of Milos is a notable source of obsidian.
Finding Tips
Volcanic Regions
Focus your search in areas with recent or geologically young rhyolitic volcanic activity. Look for lava flows, volcanic domes, and areas where volcanic glass is known to occur.
Outcrops and Stream Beds
Obsidian can be found in primary outcrops where lava flows have cooled, or as weathered fragments in stream beds and alluvial deposits downstream from volcanic sources.
Conchoidal Fracture
Look for rocks with a characteristic conchoidal (shell-like) fracture, which is typical of obsidian. The 'snowflakes' will be visible on broken surfaces or polished specimens.
Safety Precautions
Obsidian edges can be extremely sharp, comparable to surgical scalpels. Always handle with care, preferably with gloves, to avoid cuts. When collecting, wear appropriate protective gear, including sturdy boots and eye protection. 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
Rainbow Obsidian
Obsidian with Magnetite Nanoparticles
Also known as: Sheen Obsidian
Scientific Classification
- Mineral Class
- Not a true mineral, but a volcanic glass (mineraloid). The spherulites are minerals (e.g., cristobalite, feldspar).
- Group
- Volcanic Glass
- Crystal System
- Amorphous (spherulites are microcrystalline, typically tetragonal for cristobalite or triclinic for feldspar)
- Chemical Formula
- Predominantly SiO₂ (70-75%), with varying amounts of Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O, K₂O, and H₂O.
- Composition
- Silica-rich volcanic glass with embedded microcrystalline aggregates of silica polymorphs (like cristobalite) or feldspar.
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