The Enigmatic Boulder

The excavations in the croft field have unearther a granite boulder that appears to be "wedged" in place inside the building we are excavating. It does not have a flat top and it is relatively small (see picture). Does anyone out there have any ideas as to what its function was? One idea (my wifes) was that it could have been put there for boars to rub their tusks against to wear them down a bit. Alternatively it was for put there as a licking stone for added mineral content. Any other ideas? Please post comments1