Onyx is indeed a Rock-type Pokemon, although in the Gold/Silver version of the game, you can use something to may him evolve into a Metal-type or something. I haven't played Pokemon since the late 90's.
m-w.com:
Main Entry: on·yx
Pronunciation: 'ä-niks
Function: noun
Etymology: Middle English onix, from Middle French & Latin; Middle French, from Latin onych-, onyx, from Greek, literally, claw, nail -- more at NAIL
: a translucent chalcedony in parallel layers of different colors
Does this guy use the mandable claw?