The round stone at Bosporthennis gets a mention in Naenae Cornubia (1872) by William Copeland Borlase.
page 67.

Some old chap remembered a miller caled Jan from Polmear who came up to work the stone into a round shape.

The story rambles on a bit but I will post it if I find time. Borlase concludes that the stone must have been roughly circlular and did find splintered pieces lying around.

The is a similar stone at Stowes Pound in Cornwall - carved from a natural stone also abandoned .

http://www.southeastcornwall.co.uk/bodmoor.htm

PhiL