Indeed, metals common to the Final Fantasy franchise can be found, though with the same rarity as you could expect from metals in the real world (ie, finding iron is not so hard, finding gold on the other is a trial in itself). Rarer metals can be found but in, of course, smaller quantities and for the most part they have to be bought. Mythril and Orichalcum are found in the Mythril Mine but are extremely rare; so it is unlikely they can be bought at the moment, it would ask a lot of months of work before a character can pretend having enough money to do that.

Crystals can be obtained solely by characters with the final skill of the Thief Job. Metals from other worlds are not present in Crystallis, even if there are characters representing said world.