There is Power Gem and Weather Ball. And Weather ball isn't a bad choice since it has with sandstorm 100BP and 10PP.Hmmm a Special Attack using Rock-type would be pretty interesting. Too bad its best move is HP Rock or AncientPower lol. That's why I picked Fighting as a second type....to at least give it some kind of useful options like Aura Sphere etc.
I voted for Ghost mostly because I knew that Mono rock i going to win. And I liked the idéa of combining "the materia type" and the "anti-materia type".
Other than that I think that ghost as a 2ndtype would not outshine the main type, rock, as all the other would do (*looking at fightings rather good special attacks*)
And I like the idé of a pokemon with Omnious Wind, Ancientpower and Technician. Although they have vary low PP. But then again, we have weather ball as a good move.