@christibrany said
"Can someone explain basically why swords couldn't be earth, esp. as they include primarily earth minerals/metal?"
Hmmm.... how shall I speak truth and be certain it won't become lie to the ear under-developed to such things? The sword unfortunately can cut both ways. Training in swordsmanship is required by the SCA swordsmith who along with my witness and touch crafted my first sword for me. He doesn't sell swords, he makes them and sells them only after you participate in three hours (three days, one hour each) training in handling the sword - because it litterally will cut both ways.
Symbols are indeed arbitrary. Create your own if you like. Test them out. See what happens. But the symbolism of the sword representing the mind (tarot) which includes reasoning, philosophy, law and order topics, opinions, facts, mentality, is a very good choice as is. The disk, or pentagram, is a great symbol for Earth because of the paten; the dish that holds the wafer of the Eucharist is the Bringing to Earth the Spirit Made Flesh. Oh, and coins are nothing but materialism. So there's that.
But as I said, it's all arbitrary. We chose them, they didn't arrive this way. Experiment as thou wilt and then observe the report.