The premium currency Jewels is used to increase the maximum improvement level of the craftsmen. With jewels, you can heal your heroes instantly so that you can send them on further missions without waiting. You can use them to open the golden chest, and the last storage and sales slots can be purchased with jewels. Jewels are also used to improve Hero skills. Green heroes are purchased with jewels, and to upgrade green heroes to blue heroes, you will also need jewels.
Jewels are random rewards from bosses and dungons, they are given in the daily quest and log-in, can be purchased in the guild shop, and can be obtained from the silver box. In duels, jewels are awarded randomly depending on victory and defeat and the league. In adventures, jewels can be found randomly in items or received from defeated opponents.