igneous
Dark basaltic rock, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, with dull to subtly glassy luster; Mohs hardness about 5–6, no obvious cleavage as a whole, and specific gravity around 2.8–3.0. The reddish-brown areas likely represent iron-oxide weathering or alteration.
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Physical properties
Typically dark gray to black, fine-grained, with dull to subtly glassy luster; Mohs hardness about 5–6, no obvious cleavage as a whole, and specific gravity around 2.8–3.0. The reddish-brown areas likely represent iron-oxide weathering or alteration.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions or ancient flood-basalt events. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces can be used decoratively. Ordinary hand specimens generally have little market value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Vesicles, if present, may later fill with zeolites, calcite, or quartz.
Field identification & locations
Identify it by its heavy, dark, fine-grained texture and lack of visible large crystals; a streak test, hardness test, and inspection for vesicles would improve confidence. Common sources include volcanic regions, river gravels, and oceanic terrains.
