Potboilers are commonly found in Iron Age sites. They are usually smooth beach pebbles which have been used to heat water for a variety of purposes. The stones were heated in the fire and plunged into the cold water. Eventually, the dramatic changes of temperature would make them crack and they would be thrown away. That is why we find so many of them on archaeological digs of Iron Age sites.
This particular potboiler stone was found during a dig of a roundhouse on the hill behind Gairloch Museum. It is interesting because it was not just a simple beach pebble. Signs of use show that it had been a polisher stone before it was used as a potboiler. It was probably used to smooth skins and is worn smooth on its underside. The three pieces of it were found together, suggesting that it lay undisturbed for two thousand years before it was rediscovered.