NPR's guide to the running gags from the show.
Buster has a giant rock, a souvenir from his time studying.
2 episodes feature this joke:
Buster will collect a giant rock from the archeological dig.
Sources: Joke data compiled by Adam Cole / NPR. Episode metadata from The TVDB via a Creative Commons Attribution 3.0 United States (CC BY 3.0 US) license.