igneous
Dark gray fine-grained rock, possibly basalt
Basalt; primarily plagioclase feldspar and pyroxene, with possible olivine or magnetite
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, dull to subtly vitreous, fine-grained and massive; crystals are usually microscopic, with poor or absent cleavage. The specimen appears dense and rounded, though lighting limits confident identification.
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Physical properties
Typically Mohs 5–6, dark gray to black, dull to subtly vitreous, fine-grained and massive; crystals are usually microscopic, with poor or absent cleavage. The specimen appears dense and rounded, though lighting limits confident identification.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic flows and oceanic crust. It can range from Precambrian to Recent in age; age cannot be determined from the image.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railroad ballast, and dimension stone; attractive polished pieces are collected but are generally low-value.
Geological facts
Basalt commonly contains abundant iron- and magnesium-rich minerals and may be weakly magnetic if magnetite is present. Vesicles, if later filled with minerals, can produce amygdaloidal basalt.
Field identification & locations
Look for a uniformly fine texture, high density, dark color, and possible magnetic response; a streak test, hardness test, and fresh broken surface help distinguish it from slag or dark sedimentary rock. Common worldwide, especially volcanic regions and oceanic islands.
