Devitrified Obsidian
Volcanic glass with cristobalite/feldspar spherulites (SiO2-rich)
Obsidian that has partly crystallized over time, growing pale spherulite clusters within the black glass, as in snowflake obsidian.
- Mohs hardness
- 5-6
- Color
- Black to brown glass with grey or white crystal patches
- Type
- igneous
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Overview
Devitrified Obsidian is volcanic glass that has begun to crystallize, or 'devitrify,' after solidifying. The most familiar example is snowflake obsidian, where greyish-white spherulites bloom against the black glass.
Obsidian is metastable, meaning the glassy state is not its lowest-energy form. Given enough time, heat, and water, the glass slowly reorganizes into tiny radiating crystals of cristobalite (a silica polymorph) and feldspar, producing the characteristic spotted or flowery patterns.
The degree of devitrification varies from a few scattered snowflakes to material that is mostly crystalline, blurring the line between true obsidian and stony rhyolitic rock.
Formation & geology
Obsidian forms by the rapid chilling of high-silica lava into glass. Over geologic time that glass is unstable and tends to crystallize from the inside out, nucleating around impurities or gas bubbles.
Devitrification produces spherulites: tiny spheres of needle-like crystals (typically cristobalite and feldspar) that grow radially. In snowflake obsidian these appear as white-grey clusters; in spherulitic obsidian they can form larger orbs.
The process is accelerated by trapped water and mild reheating. Devitrified obsidian is found in older obsidian flows worldwide, with snowflake material common from the western United States and Mexico.
How to identify it
Look for black or brownish glass interrupted by greyish-white, snowflake-like or rounded crystal patches. The spots have a duller, granular texture compared to the glossy glass around them.
The glassy portion shows conchoidal fracture, vitreous luster, hardness 5-6, and a white streak; the crystallized patches may feel slightly softer or rougher. Under magnification the spherulites reveal a radiating fibrous structure.
Distinguish it from flower agate (chalcedony with sprays inside translucent quartz, harder at ~7) and from porphyritic obsidian (which has distinct phenocrysts rather than radiating spherulites). The combination of black glass plus grey radial blooms is diagnostic of snowflake-type devitrified obsidian.
Uses & significance
Devitrified obsidian, especially snowflake obsidian, is widely used for tumbled stones, cabochons, beads, and small carvings. The contrast between black glass and pale snowflakes makes attractive, affordable jewelry.
It is a popular lapidary material because it is common and takes a good polish. Spherulitic pieces are sometimes collected as curiosities for their orb patterns.
Metaphysically, snowflake obsidian is associated with balance, calm, and grounding. These associations are cultural beliefs rather than scientifically demonstrated properties.
Frequently asked questions
What does 'devitrified' mean?
It means the glass has partly crystallized. Obsidian glass is unstable over long timescales and slowly grows tiny crystals, turning clear black glass into spotted or flowery material.
Is snowflake obsidian a devitrified obsidian?
Yes. The white 'snowflakes' are spherulites of cristobalite and feldspar that crystallized within the black glass, the classic example of devitrification.
What are the white spots made of?
They are spherulites, radiating clusters of cristobalite (a silica mineral) and feldspar that grew inside the obsidian over time.
Is it softer than regular obsidian?
The glassy parts are the same 5-6 hardness, but the crystallized patches can be slightly softer or more granular, sometimes plucking out during polishing.
Devitrified Obsidian guides
In-depth guides for identifying, valuing, and understanding Devitrified Obsidian.
Devitrified Obsidian identified by the community
Real specimens identified with Rock Identifier.
