igneous
Tumbled dark igneous rock, possibly gabbro or basalt
Likely mafic rock composed chiefly of pyroxene and plagioclase; exact identification requires testing
AI-generated identification · may be incorrectDark gray-black, rounded and polished, with a faint granular/crystalline texture and glassy highlights. Likely Mohs hardness 5–6, vitreous to dull luster, no obvious cleavage, and relatively high specific gravity (~2.8–3.1).
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Physical properties
Dark gray-black, rounded and polished, with a faint granular/crystalline texture and glassy highlights. Likely Mohs hardness 5–6, vitreous to dull luster, no obvious cleavage, and relatively high specific gravity (~2.8–3.1).
Formation & geological history
Probably formed from cooling mafic magma: basalt if fine-grained and volcanic, or gabbro if visibly coarse-grained and intrusive. Such rocks range from ancient Precambrian specimens to geologically young volcanic material.
Uses & applications
Used as aggregate, road stone, landscaping material, and occasionally as a tumbled decorative stone; generally low jewelry value.
Geological facts
The image alone cannot reliably distinguish basalt from gabbro or other dark rocks such as diabase. A strong magnet response may indicate abundant magnetite, but many specimens are only weakly magnetic.
Field identification & locations
Common in oceanic crust, volcanic regions, and mafic intrusions worldwide; examine grain size, test streak and hardness, and use a hand lens. No reaction to dilute acid is expected.
