igneous
Dark granitic rock, likely granodiorite
Medium-grained granitoid; probably biotite granodiorite or quartz diorite
AI-generated identification · may be incorrectGray-blue, massive, granular texture with interlocking crystals of plagioclase, quartz, and dark biotite/amphibole. Likely Mohs hardness 6–7, vitreous to dull luster, no obvious cleavage as a whole rock, and specific gravity about 2.65–2.75.
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Physical properties
Gray-blue, massive, granular texture with interlocking crystals of plagioclase, quartz, and dark biotite/amphibole. Likely Mohs hardness 6–7, vitreous to dull luster, no obvious cleavage as a whole rock, and specific gravity about 2.65–2.75.
Formation & geological history
An intrusive igneous rock formed by slow cooling of silica-rich magma deep in the crust, producing visible crystals. Its exact geological age cannot be determined from the photograph; granitoids in the southern Andes and nearby Sierras range from Paleozoic to Cenozoic.
Uses & applications
Used as crushed aggregate, road stone, building stone, monuments, and occasionally polished slabs. This ordinary rounded specimen has little commercial collector value.
Geological facts
The abundant pale crystals and peppery dark minerals favor granodiorite over basalt, which is usually much finer-grained. The rounded edges indicate stream or surface weathering rather than its original formation shape.
Field identification & locations
Identify it by its visible interlocking grains, gray color, and inability to scratch easily with a knife. The reported location near 22°S, 63.7°W is compatible with eroded granitoid or metamorphic basement-derived cobbles, though a field test or thin section is needed for certainty.
