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@jim mcnamara it looks like he was able to upload it now.
@ReevesN76R -- Welcome to the PF. Can you take a closeup picture of the marks on the rock? Are those marks (like drawn), or are they something embedded?
Looks like a porphyritic basalt.
Large phenocrysts are plagioclase feldspar, groundmass is typically very fine grained olivine and pyroxenes.
They are embedded.
yes, that was my diagnosis as well
are you happy with the answer ?
The porphyritic basalt seems to be correct. I have been collecting rocks and minerals for 55 years.
a rhombic porphyry most likely - how come you found it in Minesota, tough? They are more common in the scandinavian region around Oslo, Norway.
The rock is rounded and river worn, so it was not taken directly from an outcrop. It has probably been carried by a glacier or an ice sheet during the last glacial period. It may have traveled a long way south from some volcanics in Canada.
I have found identifiable rocks in unexpected places, but only where glaciation was available to transport them.
The Great Lakes region is rich is rocks and ancient volcanism.
Just over a billion years ago the continent was on the verge of breaking up,
It looks like what I have known as a "pudding stone"
Pudding stone is a sedimentary "conglomerate" of rounded pebbles in a fine matrix. It looks like a cut slice through a plum pudding.
The rock pictured was volcanic, having a fine green basalt matrix with large phenocrysts of feldspar. A rock with phenocrysts is described as a porphyry, or as being porphyritic. The phenocrysts are crystallised out of the melt over many years as the magma gradually rises up towards a volcanic vent. The fine grained matrix indicates that it cooled quickly once it reached the surface which is why it is known to be a volcanic rock.
Does the rock have any value?
As is, not really worth much.
If you like rock cutting and polishing, it would sell for a fair amount after it was polished on a cut through the middle.
Go to a local natural history museum store or one of the places that sell display quality minerals. You can get a good idea. Some mineral collectors use a rock tumbler only to polish. No cut.
My wife did this for fun and the results were amazing.
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