Gem
You gain gems by staking a card. The color of the gem you gain depends on the color of the card you stake (when staking rainbow cards like Rainigon you can choose which color gem you want to gain). To play a card from your hand you need to have the required gems that are indicated on the card. There are also certain cards that give you a gem without staking, like Penguin Mage, Tree Mage, Money Print and Rainbow Lotus.