If I remember correctly, Magic in FFVII is more 'Nature' or 'Chemistry' than actual magic. Maybe it has something to do with Jenova and being temporarily worshiped by the Ancients. I mean it just kind of drops parts of itself so who knows what happens when those alien bits end up in the Planet's life system cycle. Maybe that's why it's housed in their temple which they built to be a really ingenious deathtrap.

I always just thought of it as using a kind of extraterrestrial nature than a terrestrial one (Fire, Bolt, etc.).