igneous
Tumbled gabbro (likely larvikite or related mafic rock)
Coarse-grained mafic intrusive rock; chiefly plagioclase feldspar, pyroxene, and minor biotite/amphibole
AI-generated identification · may be incorrectDark gray-black, rounded and polished, with scattered pale feldspar crystals and a speckled texture; generally Mohs 6–7, vitreous to dull luster, granular crystal structure, and no obvious cleavage as a whole rock.
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Physical properties
Dark gray-black, rounded and polished, with scattered pale feldspar crystals and a speckled texture; generally Mohs 6–7, vitreous to dull luster, granular crystal structure, and no obvious cleavage as a whole rock.
Formation & geological history
Formed by slow cooling of iron- and magnesium-rich magma deep in the crust, producing interlocking crystals; likely Paleozoic or older, though age cannot be determined from the photograph.
Uses & applications
Used as crushed stone, dimension stone, countertops, monuments, and decorative/tumbled specimens. Collecting value is modest unless it is a distinctive variety such as blue-flash larvikite.
Geological facts
The specimen’s rounded shape and uniform polish indicate human tumbling; its visible grains distinguish it from basalt, which is typically fine-grained.
Field identification & locations
Identify it by its dark coarse granular texture, pale feldspar, high density, and lack of layering or fossils; common sources include India, Norway, Brazil, and Madagascar. A precise identification requires an unpolished surface or mineralogical testing.
