igneous
Orbicular jasper (ocean jasper)
Silicified volcanic rock, chiefly microcrystalline quartz/chalcedony with feldspar, zeolites, and iron-oxide-rich orbicules
AI-generated identification · may be incorrectTypically gray-green to cream with concentric yellow, brown, or reddish spheres; waxy to vitreous luster, opaque, massive, and no cleavage. Hardness is about 6.5–7 Mohs; specific gravity approximately 2.5–2.7.
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Physical properties
Typically gray-green to cream with concentric yellow, brown, or reddish spheres; waxy to vitreous luster, opaque, massive, and no cleavage. Hardness is about 6.5–7 Mohs; specific gravity approximately 2.5–2.7.
Formation & geological history
Formed when silica-rich fluids silicified rhyolitic volcanic material and produced concentric mineral growths in cavities or flow textures, likely during Miocene volcanic activity. Classic ocean jasper comes from coastal Madagascar, especially near Marovato.
Uses & applications
Cut and polished for cabochons, carvings, beads, spheres, and display specimens; it has little structural or industrial use. Quality pieces are valued for vivid, well-defined orbicular patterns.
Geological facts
The pictured polished elongated piece strongly matches orbicular ocean jasper, though similar orbicular rhyolite or jasper varieties can look alike. The orbs commonly consist of chalcedony, quartz, feldspar, and iron-rich minerals.
Field identification & locations
Identify it by its repeating concentric orbicules embedded in a fine-grained jasper matrix and by its opaque, glassy polish; it should not fizz in dilute acid. Common sources include Madagascar, Mexico, Australia, and Oregon.
