Gems are a premium currency that can either be purchased or collected from some types of chests and quests.
Free chests sometimes contain gems. Many quests also give gems as a completion reward, along with coins and stickers.
Gems can be used to purchase stickers and chests in the shop. Gems can also be used to speed up the unlocking of awarded chests.
Gems are needed to upgrade certain benefits, such as how many coins there are in free chests, awarded chests or as reward for completing quests.
Your account can hold a maximum of 6,000 gems.