Acorn

Acorns were the seeds of oak trees. (TV: The Android Invasion, In the Forest of the Night)

In 2050, by which time oak trees were extinct, a Centuripede gave acorns from Robin Hood's time to Starkey and K9. (TV: The Last Oak Tree)

On Albert, Christina Morgenstern was given three golden acorns which could grant wishes by the wodewose when she answered his riddles. (PROSE: Grimm Reality)

In the far future, Hade believed that "there's one rotten acorn in every barrel" was an old Earth saying. (TV: The Sun Makers)

They could be used in coordinates. In the 51st century, River Song gave her coordinates as "triple-7 5/349910012/acorn". (TV: The Time of Angels)