Puns always lose a lot in the translation.
Petra actually means *both* rock and stone, but is of feminine gender. So there is no way
Christ could have dubbed his disciple "
Petra" without making him the subject of ridicule and sophomoric hee-hawing (a la "a male disciple named
Sue"). So he called him Petros (of masculine gender) instead, which also means *both* stone and rock (check out *
Henry George Liddell,
Robert Scott, A Greek-English Lexicon*).
But being a pre-Christian kinda guy, I'd like to point out that Yeshua
Ben Panthera was not being all that original or witty for that matter;
Homer used the name Petros for warriors in the Iliad (16.734) centuries before Yeshua was born :)