It looks like Stukeley's tiddly stone from the rare postage stamp sized 'The tiny Antiquities of smaller Druids' (Published 1742).

If it is .. then I suspect it could be part of the 'Chip Stone' mentioned by Colt-Hoare's in his Stonehenge rubble article; discovered at Stourhead during the 1982 Easter Bunny egg hunt by Master Nial Pucoje aged 7 and recognized as a Neolithic exfoliation stone once used with an early form of Badedas shower gel by early cleansers.

I suspect it could fetch at least 70p + postage.

Maybe he'll swap it for my Hitler diary. I got it from a bloke at Avebury.