igneous
Orbicular rhyolite (orbicular jasper-like volcanic rock)
Orbicular rhyolite; predominantly quartz, alkali feldspar, plagioclase, and volcanic glass, with concentric spherulitic growths
AI-generated identification · may be incorrectHardness about 6–7 Mohs; pale gray-blue to cream with rounded orbicules in a finer matrix, dull to slightly waxy luster, and no obvious cleavage. The orbicules are radial or concentric mineral aggregates formed during crystallization.
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Physical properties
Hardness about 6–7 Mohs; pale gray-blue to cream with rounded orbicules in a finer matrix, dull to slightly waxy luster, and no obvious cleavage. The orbicules are radial or concentric mineral aggregates formed during crystallization.
Formation & geological history
A silica-rich volcanic rock formed when rhyolitic lava cooled, allowing repeated mineral growth around nucleation centers; typically Paleozoic to Cenozoic, depending on locality. Common sources include Oregon and other volcanic regions worldwide.
Uses & applications
Primarily a collector and lapidary stone; it can be polished for cabochons, carvings, and decorative objects, but is rarely used structurally.
Geological facts
Orbicular textures are uncommon and highly prized for their distinctive “eye” or snowball patterns. It may be marketed under trade names such as orbicular jasper, though true rhyolite is volcanic igneous rock.
Field identification & locations
Identify it by rounded, embedded orbicules, a hard fine-grained matrix, and volcanic texture; a steel file should scratch weakly or not at all. Confirm with a hand lens and locality information, since sedimentary conglomerates can look similar.
