Humm. Jewel farming you say? Well, if you don't do you're story quest, you're missing out on big bucks. If you use the Jewel Quest guide on KHUXwiki it'll tell you where the gems are located throughout the many amounts of quest. What I usually do is run through to unlock multiple quest then pick at the ones with jewels in them. You'll see a pattern of jewel quest going by 5, but later on they won't as much which is why you need the guide. Sadly jewel farming does end and right now I'm waiting for new quest to arrive. The 5th anniversary came up and I stock piled on the powerful medals that I got with the jewels that were time consuming to get. It's all worth it because you can advance in a quick amount of time with a good amount of those jewels. You can also do it in proud mode too. It's such a bummer that jewel events don't come out often.
If you finished the quests already then what I said didn't seem helpful. Hahaha.
Anyways, good luck.