igneous
Vesicular basalt (likely iron-rich basaltic river cobble)
Basalt, predominantly plagioclase, clinopyroxene, olivine, and iron-titanium oxides
AI-generated identification · may be incorrectDark reddish-brown to gray, fine-grained, dull to slightly earthy luster, with abundant weathered vesicles and possible iron-oxide staining. Hardness about 5–6 Mohs; no obvious cleavage; typically dense, though vesicles lower specific gravity to roughly 2.6–3.0.
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Physical properties
Dark reddish-brown to gray, fine-grained, dull to slightly earthy luster, with abundant weathered vesicles and possible iron-oxide staining. Hardness about 5–6 Mohs; no obvious cleavage; typically dense, though vesicles lower specific gravity to roughly 2.6–3.0.
Formation & geological history
An extrusive igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface; the holes are gas bubbles trapped during eruption. The specimen is likely geologically young compared with continental rocks, then rounded and weathered in a stream or shoreline environment.
Uses & applications
Common basalt is used as crushed aggregate, road base, riprap, railroad ballast, and dimension stone; attractive polished pieces may be sold as inexpensive specimens. This cobble has limited jewelry value unless it proves to contain unusual minerals.
Geological facts
The reddish coloration commonly results from oxidation of iron-bearing minerals. Vesicles may later fill with zeolite, calcite, agate, or quartz, producing amygdaloidal basalt.
Field identification & locations
Identify by its fine texture, magnetic response, heavy weight, and irregular vesicles; a streak, fresh broken surface, and acid test help distinguish it from slag. Common worldwide, especially volcanic islands, oceanic crust, and basaltic river gravels.
