The symbol's got nowt to do with copper mining & probably has nowt to do with them there masons. As 4Winds says, it's a chi-rho cross, which would almost certainly mean that the stepped-base of the carving was stood upon, and from which a christian orator waffled their garbage to the villagers. The folklore of the stone originally coming from the local village green implies even more for it to have stood by the local market/village cross - precisely where them there christian weirdoes waffled their weird words...

Outta curiosity: is there a link between the feast day of St. Luke (the churchyard it's now in) & any seasonal festivity on the said village green...?