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...Is that a lump of water-worn foam/plastic? Cause it sure looks like it. If it isn't, and actually got dumped there by wave/ storm action, uh... I guess it could be a badly water-worn marble boulder or something?

Good observations.

But... it is a boulder-sized block of rock.

It is locally-sourced. And there is no marble nearby... but excellent observations! The red-stained areas help provide additional clues...

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