It is difficult to obtain rare Curios without the use of premium currency. Save as many coins as possible by replaying previously complete stages repeatedly. After accumulating a few hundred coins, go to the stages where two stars and better monsters are encountered. You then have to rely on luck to get a rare Curio. Earn gems by referring friends. For your first referral, you will be awarded a two star Curio and 50 gems. Keep referring friends, and once you reach fifteen you will get 200 gems. Once you have 300 gems you can purchase a Premium Disc for one Curio. You can also earn free gems by completing the in-game offers or watching advertising videos by selecting the "Free Gems" button in the store.