igneous
Polished river pebble, likely porphyritic volcanic rock
Likely porphyritic basaltic-andesite or altered basalt; exact identification requires a fresh surface and testing
AI-generated identification · may be incorrectFine-grained, dark brown to gray matrix with abundant pale feldspar-like crystals and reddish-brown alteration spots; dull to faintly vitreous luster, massive texture, no visible cleavage. Estimated hardness 5.5–6.5 Mohs and specific gravity about 2.7–3.0.
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Physical properties
Fine-grained, dark brown to gray matrix with abundant pale feldspar-like crystals and reddish-brown alteration spots; dull to faintly vitreous luster, massive texture, no visible cleavage. Estimated hardness 5.5–6.5 Mohs and specific gravity about 2.7–3.0.
Formation & geological history
An extrusive volcanic rock formed when lava cooled relatively quickly, preserving larger early-formed feldspar crystals in a fine groundmass; alteration likely developed through later weathering or hydrothermal fluids. Geological age cannot be determined from the photograph.
Uses & applications
Used mainly as a decorative tumbled stone, landscaping pebble, or teaching specimen; not generally suitable as a gemstone or high-value ornamental material.
Geological facts
The rounded shape and smooth surface indicate prolonged abrasion in a river, beach, or gravel deposit. The pale crystals may be plagioclase feldspar, while rusty patches commonly represent iron-oxide weathering.
Field identification & locations
Look for a dense, nonporous stone with scattered pale phenocrysts, a streak that is usually pale gray, and no reaction to dilute acid; a steel file should scratch it only slowly. Common in volcanic terrains and river gravels worldwide.
