LogFAQs > #968976389

I used a mix of these two sources:

This one mostly lists the right UM to use:
https://docs.google.com/spreadsheets/d/1_a2F1ka3xvUKHvtmo5TzqLLAtZd7GQrOLvuoDUzN_Go/edit#gid=1872576528

If the UM seems like a pain to farm (low level, bad area to fight), I checked this as a secondary:
https://www.rpgsite.net/feature/13113-xenoblade-chronicles-3-gems-guide-best-gems-how-to-perform-gem-crafting-and-where-to-find-materials

I also wrote some miscellaneous advice earlier:
https://gamefaqs.gamespot.com/boards/8-gamefaqs-contests/80154559/967909943

I only went for the X level gems so you'll have to figure something out if there's some lower Gem level material that is hard to get.