Fast gems in Two Towers GBA game?

Hey there, I've recently been playing The Two Towers on GBA. I can't see a subreddit for that game specifically, so I thought this may be a good place to ask.

I'm on postgame/new game+ and looking to farm gems to purchase whetstones and attribute points etc - is there a consensus on a fast way to do it, other than just grinding the game and selling everything? As far as I know, difficulty settings don't affect gem drops. Thanks.