Board Thread:Questions and Answers/@comment-188.63.184.161-20150113194102/@comment-25179062-20150113201258

You can obtain gems through: (1) completing regions in the Quests; (2) completing certain Special Events; (3) gifts for logging into the game daily; (4) purchasing them from the Shop.

Each unit's wiki page will show you required to evolve the unit to the next level. For example, to evolve Goth Idol Kikuri from 5* to 6*, you need 1 Miracle Totem, 2 Dark Totems, 1 Dark Idol, 1 Dragon Mimic, and 500,000 zel.