I just seen on the final fantasy wikipidia that there are these two materias you can get from hacking or using a cheat device. Booster, which is pretty much null earth, causes earth attacks to cause 0 damage. While the other, which is more odvious, means you won't get hurt from those spikes in Gi Cave. These materias are built in the game but you just can't get them in story.
I've never been a fan of cheating devices but lately I've been wanting to exp with them to get some things. Aeris, Sephiroth in FF7 as well as these new materias. Beatrex from ff9, edea and seifer from ff8 are a few. Although I'd have to say I wouldn't use them for leveling up my characters and what nots. I like the exploring and grinding. Though it would be cool to get those characters.