I've just had a quick look at the link - the light on the Cove stone does give that feature a smooth almost 'millstone' appearance. I've heard it suggested that because of the size of the Cove Stones (there is as much below the ground as above) they may have already been in their present position when they were raised. A smooth area and indentation on any sarsen was probably caused by water, I have seen quite few local stones that have these features. I have also seen ammonite fossils placed in the walls of buildings and they are almost certainly chosen for their rarity, as perhaps they were back in Neolithic times along with white quartz and stones with pre-existing rock art (Long Meg) etc.