An old tooth can be found while digging at the digsite using a trowel obtained from the Varrock Museum or an Examiner (A gardening trowel will not work.) on the digging area west of the animal skull or panning in the panning area with a panning tray. The archaeological expert says, "Oh, an old tooth. It looks like it has come from a mighty being. Pity there are no tooth fairies around here!" There are currently no uses for the old tooth. It is considered a useless item because it can't be traded, gives no coins when alchemised, and can't be used in any way.
For an exhaustive list of all known sources for this item, see here