Spherulitic Obsidian Identification Guide
Identify spherulitic obsidian (snowflake-type volcanic glass) by its glassy black base, radial crystal spheres, conchoidal fracture, and low density.
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What Spherulitic Obsidian Looks Like
Spherulitic obsidian is volcanic glass in which clusters of small, radiating crystals — spherulites — have grown within the glass during slow cooling and partial devitrification. The base is the classic glassy black (or brown-black) obsidian, dotted with grayish-white, tan, or reddish rounded patches. When the spherulites are white and snowy, the rock is sold as snowflake obsidian.
- Color: black to brown-black base with white/gray/tan/red spherules
- Luster: vitreous (glassy) on the glass; the spherulites are duller, feldspar/cristobalite radial growths
- Transparency: translucent on thin edges to opaque
- Habit: amorphous glass with embedded radial spherulitic clusters
Step-by-Step Field-ID Checklist
- Confirm it is glass. Look for a bright, glassy luster and conchoidal (shell-like, curved) fracture with razor edges.
- Examine the spheres. Spherulites are rounded, often with a faint radial or feathery internal structure — they are crystals growing outward from a center, not bubbles or inclusions sitting on top.
- Note the contrast. Lighter spherulites stand out sharply against the dark glassy matrix.
- Check translucency on a thin edge — held to light, obsidian glows at the margins.
- Heft it — it feels light, like glass, not like a dense crystalline rock.
Key Diagnostic Tests
- Mohs hardness: ~5–5.5 (glass); it scratches with a steel file but will scratch a copper coin.
- Streak: white.
- Fracture: conchoidal — the single most diagnostic feature of obsidian.
- No cleavage (amorphous).
- Density: low, ~2.35–2.6 g/cm³.
- Not magnetic; no acid reaction.
Common Look-Alikes and How to Tell Them Apart
- Snowflake obsidian: essentially the same material; 'snowflake' refers specifically to white cristobalite spherulites — it is a variety of spherulitic obsidian, not a different rock.
- Porphyritic obsidian: has angular feldspar phenocrysts (crystals with flat faces) rather than rounded radial spherules.
- Black glass slag (man-made): often shows trapped round air bubbles, a more even color, and sometimes a greenish tint in transmitted light; lacks natural spherulitic radial structure.
- Basalt: a crystalline volcanic rock that is dull, fully opaque, denser, and breaks unevenly — not glassy with conchoidal fracture.
- Black tourmaline or onyx: crystalline and harder; tourmaline forms striated prisms, banded onyx (chalcedony) is harder (7) and lacks glassy spherulites.
Where It Is Found
Spherulitic and snowflake obsidian occur in rhyolitic lava flows and domes where glass cooled slowly enough for spherulites to nucleate. Major sources include the western United States — Utah, Nevada, Oregon, California, Arizona (Apache region) — plus volcanic regions of Mexico, Iceland, and Armenia.
Frequently asked questions
How can you tell if it's real spherulitic obsidian?
It is natural volcanic glass: glassy vitreous luster, conchoidal (curved, shell-like) fracture with sharp edges, low density, and translucent edges, dotted with rounded radial spherulite clusters that grew inside the glass rather than sitting on the surface.
Is spherulitic obsidian the same as snowflake obsidian?
Snowflake obsidian is a type of spherulitic obsidian where the spherulites are white cristobalite 'snowflakes.' Spherulitic obsidian is the broader term and the spheres can be gray, tan, or reddish too.
What causes the spheres in spherulitic obsidian?
As the glass cooled (or later partly devitrified), tiny crystals of feldspar and cristobalite nucleated and grew outward in radiating clusters, forming the rounded spherulites embedded in the black glass.
Spherulitic obsidian vs basalt — how do I tell them apart?
Obsidian is glassy with conchoidal fracture and translucent edges; basalt is a dull, fully opaque, denser crystalline rock that breaks unevenly and shows no glassy spheres.
How hard is spherulitic obsidian?
About 5 to 5.5 on the Mohs scale, like most natural glass, so it can be scratched by a steel file and will hold a very sharp edge when broken.
Spherulitic Obsidian identified by the community
Recent Spherulitic Obsidian specimens identified with Rock Identifier.
