Well, it’s possible it’s a poo left by a terrified creature as a carnivorous duck ate it 40 million years ago, but who’s to say…
Here’s some photos I’ve taken of a few of the Avebury Sarsens, with holes left by tree roots growing up through the layer of sand before it solidified and turned to rock.
Some of the holes are conical, so it’s possible they were caused grit or pebbles being spun around by flowing water eroding the rock over many years when the rock was still lying horizontally on the ground- I’ve seen that happening elsewhere, in Wales, actually, so who can say with chambord’s pebble.
It’s a pleasing thing in its own right, I’ve got various pebbles I’ve picked on different beaches, with a variety of different colours and textures, they’re just nice things to hold, even without any fossil features. I’ve got one or two of those as well, mostly from Charmouth.