mineral
Orbicular jasper (likely ocean jasper)
Silica-rich microcrystalline quartz/chalcedony (SiO₂), with iron-oxide and possibly feldspar or zeolite inclusions
AI-generated identification · may be incorrectHardness about 6.5–7 Mohs; opaque brown, cream, and black orbicular pattern; waxy to dull luster; cryptocrystalline quartz with no visible cleavage. The rounded, polished surface appears water-worn or tumbled.
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Physical properties
Hardness about 6.5–7 Mohs; opaque brown, cream, and black orbicular pattern; waxy to dull luster; cryptocrystalline quartz with no visible cleavage. The rounded, polished surface appears water-worn or tumbled.
Formation & geological history
Formed from silica-rich volcanic material, probably by hydrothermal alteration and repeated deposition around inclusions or cavities; classic ocean jasper formed in Oligocene–Miocene volcanic rocks of Madagascar. Exact locality cannot be confirmed from the image.
Uses & applications
Used for cabochons, carvings, polished specimens, and decorative stone; too irregular and opaque for faceted gemstones. Commonly collected for its orbicular pattern.
Geological facts
Orbicular jasper is prized for concentric “eye” patterns produced by contrasting mineral growth. The pictured specimen may also be a related orbicular rhyolite or jasper breccia rather than true ocean jasper.
Field identification & locations
Identify by testing for glassy/waxy silica texture, lack of cleavage, strong resistance to scratching, and orb-like color bands; common sources include Madagascar, Mexico, Australia, and the western United States. Avoid acid and use a streak test cautiously, since alteration minerals can vary.
