igneous
Orbicular rhyolite (likely)
Silica-rich volcanic rock, chiefly quartz and feldspar with orbicular aggregates of feldspar, quartz, and mafic minerals
AI-generated identification · may be incorrectHardness about 6–7 Mohs; cream to pink-gray matrix with dark brown/purple rounded orbicules, dull to slightly glassy luster, and no obvious cleavage in the whole rock. Dense, fine-grained, and commonly rounded by stream or beach abrasion.
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Physical properties
Hardness about 6–7 Mohs; cream to pink-gray matrix with dark brown/purple rounded orbicules, dull to slightly glassy luster, and no obvious cleavage in the whole rock. Dense, fine-grained, and commonly rounded by stream or beach abrasion.
Formation & geological history
Forms when viscous rhyolitic magma cools rapidly, allowing concentric mineral-rich spherulites or orbicules to develop; usually Cenozoic to older volcanic age depending on locality. The specimen appears water-worn, but its exact locality cannot be determined from the image.
Uses & applications
Used mainly as a decorative stone, lapidary material, and collector specimen; attractive pieces may be cut and polished for cabochons or display.
Geological facts
Orbicular textures are uncommon and can resemble leopard jasper, although true orbicular rhyolite is igneous rather than sedimentary. The circles are mineral growth structures, not fossils.
Field identification & locations
Identify by its volcanic fine-grained matrix, rounded concentric patches, hardness, and lack of sedimentary layering; a fresh surface, acid test, and thin section would improve certainty. Examples occur in volcanic regions including Oregon, California, Mexico, and Finland.
