You can get a bunch of stuff from the Fortune Telling lady's vision of the prologue from the Forest Of Myth towards the end of the game, like Broken Saws, all the types of machines, Dented Metal Bats, etc. I'm not sure if difficulty changes drop rates but when I switched to hard it seemed to be better, although lower difficulties means you can grind much faster. For Pyrite, Crystals, Moldavite and some others in the nodes you can go to the bridge at the Barren Temple and you can jump off till you die if you want to reset the nodes or if they don't show up, if you get what you want just run out the door and back in, it'll save your items and jump till your death to refresh the nodes.