Spherulitic Obsidian
Volcanic glass (SiO2-rich) with radial spherulites
Obsidian containing spherulites — small radiating spheres of feldspar and cristobalite that crystallized within the cooling volcanic glass.
- Mohs hardness
- 5-6
- Color
- Black to gray with white, gray, or tan rounded radiating clusters
- Type
- igneous
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Overview
Spherulitic obsidian is volcanic glass that contains spherulites: roughly spherical, internally radiating aggregates of needle-like crystals, usually alkali feldspar and the silica mineral cristobalite. They appear as round, often star-like or snowflake-like patches set in the dark glass.
Spherulites form by devitrification — the slow, partial conversion of glass into crystals after the lava has solidified. As a result, spherulitic obsidian represents an intermediate state between fully glassy obsidian and fully crystalline rhyolite.
The best-known commercial form is snowflake obsidian, where grayish-white cristobalite spherulites stud a black matrix.
Formation & geology
Spherulites nucleate in silica-rich volcanic glass that cools just slowly enough, or is reheated enough, to allow limited crystal growth. Crystals grow outward from central nucleation points in radiating fibers, building spheres.
This happens within rhyolitic obsidian flows and domes, often near the interior or base where cooling lags behind the chilled glassy margins. Over time devitrification can enlarge and multiply the spherulites.
Classic localities include Glass Buttes and other Oregon flows, the Utah "snowflake" deposits, Mexico, Iceland, and Lipari in Italy.
How to identify it
Identify it by rounded, radiating gray-white or tan patches with a feathery or starburst internal texture inside a glassy black matrix. The host glass shows vitreous luster and conchoidal fracture; streak is white; hardness is about 5-6.
The spherulites are slightly softer and duller than the surrounding glass and may stand out on a weathered surface.
Do not confuse with porphyritic obsidian (discrete pre-existing crystals rather than radiating spheres) or with flecks of foreign mineral. The clearly radial, spherical clusters are the diagnostic feature.
Uses & significance
Spherulitic obsidian — especially snowflake obsidian — is widely cut into cabochons, spheres, beads, pendants, and tumbled stones, prized for the contrast between black glass and white "flowers."
It is also a teaching specimen for geologists illustrating devitrification.
Metaphysically it is marketed as a stone of purity and balance, said to bring calm; such properties are traditional beliefs rather than established science.
Frequently asked questions
What is a spherulite?
A spherulite is a small ball of crystals — typically feldspar and cristobalite — that grew radially outward from a central point inside volcanic glass.
Is spherulitic obsidian the same as snowflake obsidian?
Snowflake obsidian is the most common spherulitic obsidian, named for its white cristobalite spherulites. The term spherulitic covers all such radiating-crystal obsidians.
Why are the spheres lighter than the rest?
The spherulites are crystalline feldspar/cristobalite, which is paler and more opaque than the surrounding dark glass.
Is it valuable?
It is an affordable, popular lapidary material rather than a precious gem; value comes from attractive, well-defined spherulite patterns and a good polish.
Spherulitic Obsidian guides
In-depth guides for identifying, valuing, and understanding Spherulitic Obsidian.
Spherulitic Obsidian identified by the community
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