igneous
Weathered mafic rock, likely basalt or gabbro
Basaltic/gabbroic rock; chiefly plagioclase feldspar and pyroxene, possibly olivine
AI-generated identification · may be incorrectDark gray-green, fine- to medium-grained, dull to slightly waxy luster, massive with no obvious cleavage; typical hardness about Mohs 5–6 and specific gravity roughly 2.8–3.1. The rounded surface reflects stream or weathering abrasion.
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Physical properties
Dark gray-green, fine- to medium-grained, dull to slightly waxy luster, massive with no obvious cleavage; typical hardness about Mohs 5–6 and specific gravity roughly 2.8–3.1. The rounded surface reflects stream or weathering abrasion.
Formation & geological history
An igneous rock formed when mafic magma cooled, either rapidly at the surface (basalt) or slowly underground (gabbro); commonly Mesozoic to Cenozoic, though older examples occur. The photograph is insufficient to determine the exact variety or age.
Uses & applications
Basalt and gabbro are used as crushed aggregate, road stone, riprap, paving, and dimension stone; attractive polished gabbro is used for countertops and monuments. This small weathered specimen has little commercial value.
Geological facts
Basalt is the most widespread volcanic rock and forms much of oceanic crust, while gabbro is its coarse-grained intrusive equivalent. Green coloration may indicate chlorite or other alteration minerals.
Field identification & locations
Look for a dense, dark rock that does not fizz with dilute acid and may show tiny feldspar crystals; a fresh broken surface is more diagnostic than the weathered exterior. Common sources include volcanic terrains, river gravels, and oceanic crust.
