For a joke, I wanted to translate “Seize the rock!” into Latin, along the lines of “Carpe diem!” I know the “petra” is the world for rock, but I don’t know the appropriate grammatical form to put into the phrase. Can anyone lend a hand? Thanks in advance. 
Carpe petram.
Wow, that was fast! Thanks!
I’m always on the ball.
While you’re at it:
Vox Populi - voice of the people
E Pluribus Unum - out of many, one
Is “Vox Pluribus” a correct construction? “Many voices”?
No. If you are using “many voices” as the subject of a sentence, then you need “voces plures.” Not very elegant or euphonic, but there it is.
Romani ite domum!
Romans go home!