igneous
Likely basaltic rock or mafic metamorphic specimen
Basalt, possibly altered to amphibolite/greenstone; chiefly plagioclase and pyroxene, with possible chlorite, amphibole, and iron oxides
AI-generated identification · may be incorrectDark gray to purplish, fine-grained, massive, and dull to weakly vitreous; apparent hardness about 5–6 Mohs, with no obvious cleavage. Pale veins or patches may be quartz, calcite, or alteration minerals; exact identification requires a fresh surface and tests.
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Physical properties
Dark gray to purplish, fine-grained, massive, and dull to weakly vitreous; apparent hardness about 5–6 Mohs, with no obvious cleavage. Pale veins or patches may be quartz, calcite, or alteration minerals; exact identification requires a fresh surface and tests.
Formation & geological history
Basalt forms from rapidly cooled mafic lava, commonly in oceanic crust and volcanic regions; metamorphosed equivalents form under low- to medium-grade pressure and heat. Ages range from recent eruptions to Precambrian rocks billions of years old.
Uses & applications
Used as crushed aggregate, road stone, riprap, railroad ballast, and building stone; attractive polished pieces may be sold as landscaping or collection specimens.
Geological facts
The purple coloration can result from iron oxidation, surface staining, or alteration rather than a distinct purple mineral. The specimen’s rounded edges suggest natural weathering or tumbling.
Field identification & locations
Identify with a streak test, hardness test, magnetism check, and examination of a fresh fracture; basalt commonly has tiny plagioclase crystals and may be weakly magnetic. Common worldwide, especially volcanic belts and oceanic terrains.
