igneous
Weathered mafic rock, likely basalt
Basalt; predominantly plagioclase feldspar and pyroxene, possibly olivine and iron oxides
AI-generated identification · may be incorrectDark brown-black, fine-grained and dull to weakly vitreous, with rusty weathering; typical hardness 5–6 Mohs, no obvious cleavage, and specific gravity about 2.8–3.1. The apparent golden flecks may be weathered olivine, mica, or iron-oxide staining.
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Physical properties
Dark brown-black, fine-grained and dull to weakly vitreous, with rusty weathering; typical hardness 5–6 Mohs, no obvious cleavage, and specific gravity about 2.8–3.1. The apparent golden flecks may be weathered olivine, mica, or iron-oxide staining.
Formation & geological history
Basalt forms when low-silica magma erupts and cools rapidly at or near Earth’s surface, commonly in lava flows and oceanic crust. It may range from ancient Precambrian material to geologically recent volcanic rock; age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and occasionally dimension stone; rarely used as a gemstone except when containing attractive cavities or alteration minerals. This small weathered fragment has minimal commercial value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Its brown surface likely reflects oxidation and chemical weathering of iron-bearing minerals.
Field identification & locations
Look for a dense, dark, non-layered rock that commonly attracts a magnet weakly and is harder than glass. Common sources include volcanic regions, oceanic crust, Hawaii, Iceland, India’s Deccan Traps, and the Columbia River Basin.
