Gems can be obtained by filling the gem meter or on the Free Prizes Dashboard.
The Gem meter is filled up with every correct answer you give. You get 1 gem point each time you answer a Challenge question correctly and you get 2 gem points each time you answer a Classic Game question correctly. When you collect 50 gem points, you receive one gem. Gems can be used to get cards from machines by clicking “Get more cards” on the main game screen.
You can also obtain some gems by trading duplicate cards on the Trade Card Kiosk machine or you can buy from Shop.