There are 3 easy way of farming jewels and all of them involves getting a Jewel Pouch that contains either random jewels oran Octagonal Crystal.
(1)·You can go farm Circus's Coupon to exchange for a Sander's Hidden Jewel Pouch with Magician Sander outside the circus which requires 20 coupons per pouch. (You get a certain number of coupon with different events inside the circus. What I usually do is get 3 of my friends to do the Amazing Horse Race, you'll get 10 coupons per run with 3 run limit)
(2)·You can go farm Colosseum's Medals to exchange for a Michael's Jewels Pouch with Colosseum Shopkeeper Michael which requires 60 medals per pouch. (do some pvp rounds, every win or lose will get you some medals)
(3)·You can go farm Free Adventurer's Guild Points to exchange for a Free Adventurer's Guild Jewel Pouch with Merchant Pero which requires 150 points per pouch.(collect gifs in the dungeons and you can agift to the NPC's in Saint haven, do abyss mode with bunny keys, thats will give u 2 gifts for NPC's faster)
GOOD LUCK!