The older translations of Gerald, like that of Colt-Hoare below(*p.446), tell the following tale:

There is a stone here resembling a human thigh, which possesses this innate virtue, that whatever distance it may be carried, it returns, of its own accord, the following night, as has often been experienced by the inhabitants. Hugh, earl of Chester, in the reign of king Henry I, having by force occupied this island [1096 CE*] and the adjacent country, heard of the miraculous power of this stone, and, for the purpose of trial, ordered it to be fastened, with strong iron chains, to one of a larger size, and to be thrown into the sea. On the following morning, however, according to custom, it was found in its original position, on which account the earl issued a public edict, that no one, from that time, should presume to move the stone from its place. A countryman, also, to try the powers of this stone, fastened it to his thigh, which immediately became putrid, and the stone returned to its original situation.

Which certainly sounds like our wonder.