igneous
Tumbled orbicular granite (likely orbicular diorite)
Orbicular granitoid; chiefly quartz, feldspar, and dark amphibole/biotite minerals
AI-generated identification · may be incorrectPolished, rounded specimen with gray-green to black mottled/orbicular patterns and vitreous-to-dull luster. Typical hardness is about 6–7 Mohs; granular crystalline texture, no consistent cleavage as a whole, and specific gravity roughly 2.7–2.9.
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Physical properties
Polished, rounded specimen with gray-green to black mottled/orbicular patterns and vitreous-to-dull luster. Typical hardness is about 6–7 Mohs; granular crystalline texture, no consistent cleavage as a whole, and specific gravity roughly 2.7–2.9.
Formation & geological history
Formed when silica-rich magma crystallized slowly underground, with distinctive concentric mineral-rich orbicules developing during unusual cooling and fluid conditions. Such rocks are generally Precambrian to Phanerozoic, depending on locality; the exact age cannot be determined from the image.
Uses & applications
Used as a decorative stone, tumbled collectible, paperweight, or cabochon material; rarely used for construction because orbicular varieties are uncommon and valued aesthetically.
Geological facts
The visible rounded patches are likely orbicules or contrasting mineral aggregates rather than fossils. The photograph cannot reliably distinguish orbicular granite from orbicular diorite, gabbro, or a manufactured stone.
Field identification & locations
Identify it by its natural granular texture, interlocking crystals, and irregular dark-light orbicules; test for glassy uniformity, which would suggest imitation. Orbicular rocks occur in Finland, Madagascar, Corsica, South Africa, and scattered localities worldwide.
