Gems are a special type of item in Final Fantasy: The 4 Heroes of Light that don't take space from the inventory, but are stored in a separate menu. The player can hold maximum of 99 gems of a certain type at once. Gems are received as battle spoils from enemies and found during battle with Merchant abilities. The rarest types of gems, Diamonds, are only received from bosses.
The gems can be sold to earn money, or they can be used to learn new abilities on the characters' crowns, or used to boost the characters' equipment in a shop in Urbeth. The further the player enhances their crowns or equipment, the more and rarer types of gems are required.