This is what Craig Weatherhill says in his book ‘Cornovia: Ancient Sites of Cornwall’ (Cornwall Books, 1985, revised 1997 & 2000): “In the back garden of Tolvan Cross Cottage, on a minor road 1mile due North of Gweek. Permission to view must be sought. A large upright slab, triangular in shape, 2.3m tall and 2.2m wide at the base. The centre of the stone is pierced by a circular hole 44cms in diameter, the purpose of which is unknown. It is probably Bronze Age.”
Homer Sykes in ‘Mysterious Britain’ (Weidenfeld & Nicolson, 1993) says that it is unknown how far into the ground it stands, and that the hole is bevelled. He says that some suggest the Tolvan was used to block a now lost ancient burial chamber, and suggests that the hole allowed a way in for funeral purposes and a passage out for the spirits of the dead. An interesting idea but strange that nothing else like it exists elsewhere.