igneous
Pillow basalt (likely metamorphosed basalt/greenstone)
Basalt, commonly altered to chlorite–actinolite–epidote greenstone
AI-generated identification · may be incorrectDark blue-green, fine-grained, massive, and weakly foliated with pale mineral veins; typically vitreous to dull, hardness about 6, and specific gravity 2.8–3.1. The rounded shape is beach or stream abrasion, not original crystallization.
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Physical properties
Dark blue-green, fine-grained, massive, and weakly foliated with pale mineral veins; typically vitreous to dull, hardness about 6, and specific gravity 2.8–3.1. The rounded shape is beach or stream abrasion, not original crystallization.
Formation & geological history
Basalt forms when mafic lava cools rapidly, commonly underwater as pillow lava; later low-grade metamorphism and hydrothermal fluids can produce greenstone and quartz/calcite veins. Individual rocks may range from Precambrian to Cenozoic in age.
Uses & applications
Used as aggregate, riprap, road stone, landscaping material, and occasionally as a lapidary rough or polished decorative stone; ordinary pieces have little gem value.
Geological facts
The green color commonly reflects chlorite, epidote, and actinolite replacing original pyroxene and glass. Pale veins are likely quartz, calcite, or zeolite minerals filling fractures.
Field identification & locations
Identify it by its very fine texture, dark green-gray color, great toughness, and absence of visible large crystals; a streak, acid, and hardness test can distinguish calcite-rich veins from quartz. Common in oceanic volcanic terrains and glacial or coastal gravel deposits.
