The top bar at the stage select screen. The counter on the right is the jewel counter.
A picture of a jewel!
Jewels are premium currency that can be used in the Shop for Hearts or Coins, or recovering 5 moves/10 seconds during stages when the player has run out of moves/time. Jewels can be purchased from the Jewel Shop at a rate of about 1 Jewel/$1. They can also be obtained ingame by clearing the last stage of any area in the Main Stages (only on 3DS) or from various events and giveaways.