igneous
Basaltic beach cobble
Fine- to medium-grained basalt, likely locally porphyritic or altered
AI-generated identification · may be incorrectDark gray to black, rounded by abrasion, with scattered reddish-brown and pale mineral grains; typically dull to faintly subvitreous luster, massive texture, no obvious cleavage, hardness about 5–6 Mohs, and specific gravity roughly 2.8–3.1.
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Physical properties
Dark gray to black, rounded by abrasion, with scattered reddish-brown and pale mineral grains; typically dull to faintly subvitreous luster, massive texture, no obvious cleavage, hardness about 5–6 Mohs, and specific gravity roughly 2.8–3.1.
Formation & geological history
An extrusive igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in volcanic settings. Its exact geological age cannot be determined from the photograph; the rounded shape reflects coastal or stream transport.
Uses & applications
Used as aggregate, road base, riprap, railway ballast, landscaping stone, and occasionally architectural facing; attractive polished pieces may be sold as decorative stone. This ordinary cobble has minimal collector value.
Geological facts
The reddish areas may be oxidation of iron-bearing minerals, while pale flecks could be plagioclase or altered volcanic material. Basalt commonly contains pyroxene, plagioclase, and sometimes olivine or magnetite.
Field identification & locations
Identify it by its dark, dense, fine-grained volcanic texture, lack of layering, and possible weak attraction to a strong magnet from magnetite. Common on volcanic coasts, river gravels, and glacial deposits; a hand lens and streak/magnet tests would improve identification.
