Fire and Ice resisted the opposing elements until FF5, where they finally decided to be more intuitive and make Fire resist Fire, and so on. FF4 actually took it a step further and made Fire stuff give you Fire weakness and what not.
Similarly, Diamond Stuff gives you resistance to Lightning; this was actually kept true in FF5! FF6 removed it, though I wanna say FF12 brought it back, and I think FF12 may have even done swapped resistances for the elemental shields, but I can't say I remember offhand. FF12 elemental resistance is a blur beyond "Wind Breakers resist Wind, Black Masks Absorb Darkness, White Masks (I think?) absorb Holy", cause those are the 3 things you care about for super bosses.
This is yet another example of why FF3DS needs punting for the in game documentation. They HAD the ability to tell you crucial things like elemental resistance, but the game does not. I could forgive it in FF3o, cause back then, the idea of "in game descriptions" hadn't been thought of, so whatever. FF3DS, if they can say "Resists Status", why can't they say "Resists *element*"? Blindly equipping stuff is an artificial way to increase challenge, afterall, and is more a case of "bad polish."
Though honestly wouldn't surprise me if this was one of those "YOU MUST KEEP THE NATURE OF THE ORIGINAL GAME IN TACT!" nonsense Japanese gamers decided should be, which led to the stupid "Save Pointless Final Dungeon with the hardest boss in the game (and probably hardest final boss in the series) right at the end...and this dungeon is NOT SHORT."