After you use a Growth Potion you will have the ability to check out the Shop. Do so and if you have adequate Gems, purchase a Treasure Key and, if possible, a shield and a weapon. It does not truly matter which one you purchase, just that you purchase one. The Treasure Key will help you unlock a chest if you find one, and you can find respectable loot inside the chests! 20 Gems used to purchase that Treasure Key will certainly worth it.