metamorphic
Dark green, fine-grained pebble (likely amphibolite or greenstone)
Likely amphibolite, composed mainly of hornblende and plagioclase; exact identification requires examination
AI-generated identification · may be incorrectDark green-black, mottled, fine- to medium-grained surface with a subdued luster; likely Mohs hardness about 5–6. The pebble shape suggests water-worn rounding; crystal structure and cleavage are not discernible from the image.
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Physical properties
Dark green-black, mottled, fine- to medium-grained surface with a subdued luster; likely Mohs hardness about 5–6. The pebble shape suggests water-worn rounding; crystal structure and cleavage are not discernible from the image.
Formation & geological history
If amphibolite, it formed when basaltic rock was metamorphosed under moderate to high heat and pressure; such rocks commonly date from Precambrian or younger crustal events. The rounded form likely resulted from transport in a river or beach environment.
Uses & applications
Comparable rock is used as crushed aggregate and, when attractive and sound, for landscaping or decorative stone; this small pebble has little commercial use.
Geological facts
“Greenstone” is a broad field term for altered mafic rocks, while amphibolite is defined by its amphibole-rich composition. Color alone cannot reliably distinguish them.
Field identification & locations
The dark, mottled appearance and rounded outline are consistent with a mafic rock, but mineral grains are too indistinct here for confident identification. Common in ancient metamorphic belts and stream or beach gravels.
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