Free Emeralds Quests

As you progress through the game, you receive 5 Emerald Quests that return more Emeralds than spend:

These quests also work while discounts are active because they count how often you used the Emeralds button and not how much you have spent.

Gallery
The text at the bottom of each screen hints to these quests:

Side note
Many new players squander their Emeralds on useless things when they really should be focusing on one thing: Totem. A Totem is one of the most important things in the game, and players should get it the moment they can, which is at Team Level 60. You should definitely save up to 4,500 Emeralds until you reach Team Level 60, where you can spend them at the Altar of the Elements.