igneous
Weathered mafic rock, likely basalt or gabbro
Basalt–gabbroic rock; chiefly plagioclase feldspar and pyroxene, with possible olivine and magnetite
AI-generated identification · may be incorrectDark gray-brown, fine- to medium-grained, with a dull to locally sparkly luster from exposed crystals; typically massive, no visible cleavage, Mohs hardness about 5–6, and specific gravity roughly 2.8–3.1. The rounded shape and brown surface indicate weathering or stream abrasion.
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Physical properties
Dark gray-brown, fine- to medium-grained, with a dull to locally sparkly luster from exposed crystals; typically massive, no visible cleavage, Mohs hardness about 5–6, and specific gravity roughly 2.8–3.1. The rounded shape and brown surface indicate weathering or stream abrasion.
Formation & geological history
Formed when iron- and magnesium-rich magma cooled; basalt crystallizes rapidly at or near the surface, whereas gabbro crystallizes slowly underground. Such rocks range from ancient Precambrian material to geologically recent volcanic flows.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and sometimes building or decorative stone; generally low collector value unless unusually fresh, textured, or locality-specific.
Geological facts
The metallic-looking flecks are likely reflective feldspar, pyroxene, or weathering products rather than visible gold. A magnet may attract it if magnetite is abundant.
Field identification & locations
Common in volcanic terrains, oceanic crust, and glacial or stream deposits. Look for a dark, dense, non-layered rock; a fresh broken surface, acid test, streak, and magnet test would distinguish basalt/gabbro from slag or metamorphic rocks.
