In-App Purchases - First Purchase Gem Promotion

The first time you purchase each Gem Set in the Purchase Bundles and Gems menu, the set will include more Gems than usual. "(Gem Sale)" in a Gem Set's description indicates that the First Purchase Gem Promotion is active. After you purchase a Gem Set for the first time, its number of Gems will reset to normal.

Fortunately, the First Purchase Gem Promotion resets at the start of every month.