igneous
Orbicular rhyolite (orbicular jasper-like volcanic rock)
Silica-rich rhyolite containing concentric feldspar–quartz orbicules, commonly with iron-oxide staining
AI-generated identification · may be incorrectTypically gray to tan with brown, reddish, or cream rounded orbicules; fine-grained matrix, dull to slightly waxy luster, hardness about 6–7 Mohs, no prominent cleavage, and specific gravity roughly 2.4–2.7. The visible spheres are mineral-growth structures rather than fossils.
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Physical properties
Typically gray to tan with brown, reddish, or cream rounded orbicules; fine-grained matrix, dull to slightly waxy luster, hardness about 6–7 Mohs, no prominent cleavage, and specific gravity roughly 2.4–2.7. The visible spheres are mineral-growth structures rather than fossils.
Formation & geological history
Forms when silica-rich volcanic magma cools and crystallizes, with localized nucleation producing orbicules; the exact age depends on locality, commonly Paleozoic to Cenozoic. Weathering can enrich the spheres in hematite or limonite, creating the brown-red colors.
Uses & applications
Used mainly as a collector specimen and occasionally cut or polished for cabochons, slabs, and decorative objects; it has little structural value compared with ordinary rhyolite.
Geological facts
Orbicular textures are uncommon and can resemble conglomerate or fossil material. Similar material is marketed under names such as orbicular jasper, although “jasper” is often a trade term rather than a precise rock classification.
Field identification & locations
Identify it by rounded, internally concentric or radiating patches embedded in a coherent volcanic matrix, with no loose pebble boundaries. Notable examples occur in Oregon, California, Mexico, Madagascar, and Australia; locality information is needed for certainty.
