Which balls had Relativity? Was it just the Mastermind Einstein?
Out of the current/upcoming lineup the Inferno Blue Flame, Edge, Edge Pearl, Fanatic SS, and Magnitude 035 have Relativity coverstock variants. Then for the retired balls the Mastermind Strategy, Mastermind Einstein, Mastermind Brainiac, Melee Hook, and Fanatic had Relativity coverstock variants.
So quite a few. I didn't realize just how many actually did until I looked it up. The only ball I've had with a Relativity coverstock is the Mastermind Einstein and I really like it. It's more of a medium condition ball for me but that's fine, I have other options to go on top of it. Doesn't start grabbing too early and has nice power off the end.