Jewels can be slotted into empty sockets on equipment to gain various bonuses. You can combine jewels to power up a single jewel. You must spend gold each time you want to insert or remove a jewel. The price increases with the rarity of the jewel. You can sell unwanted jewels for gold. After a jewel reaches its maximum level, you can evolve it if you have the proper materials, which will then permanently increase its bonus stats. Also consider waiting for special events where fusions will have a guaranteed bonus.