A slab of...

...rare and exotic Italian marble?

No. The reflections of a maple tree in a sidewalk puddle, stippled and textured by the aggregate of the path, stamped with random patterns in the silt under the water (left by fallen leaves) and highlighted with scattered, tiny slush bubbles.

Thanks to Mr. F for spotting this on the dog walk and calling me out to see 'the great blip on the sidewalk' he had found!
He has been duly promoted to EBSA (Executive Blip Spotting Assistant)