igneous
Porphyritic mafic rock, likely basalt or gabbro
Pyroxene–plagioclase basalt/gabbro; exact identification requires a fresh surface or testing
AI-generated identification · may be incorrectDark gray-green, coarse to partly porphyritic texture with pale plagioclase and dark pyroxene/amphibole; likely Mohs 5–6, dull to weakly vitreous luster, little visible cleavage, and specific gravity about 2.8–3.1.
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Physical properties
Dark gray-green, coarse to partly porphyritic texture with pale plagioclase and dark pyroxene/amphibole; likely Mohs 5–6, dull to weakly vitreous luster, little visible cleavage, and specific gravity about 2.8–3.1.
Formation & geological history
Formed when mafic magma crystallized; fine-grained basalt cools at or near the surface, whereas coarse gabbro crystallizes underground. Its age cannot be determined from the photograph and depends on locality.
Uses & applications
Used as crushed aggregate, road stone, riprap, and sometimes building stone; attractive polished pieces may be sold as dark decorative stone, but this specimen has modest gem or collector value.
Geological facts
The pale crystals are probably calcium-rich plagioclase, while the dark grains are likely pyroxene or amphibole. Weathering can produce the greenish-brown surface and obscure the original texture.
Field identification & locations
Common in volcanic terrains, oceanic crust, and mafic intrusions worldwide. A fresh broken surface, magnet test, streak, and hardness test help distinguish basalt/gabbro; visible crystals and a heavy, dark appearance support a mafic composition.
