In the Jewelry Box event, you can receive an extra reward for completing a level on the first, second, or third try.
If you complete the level on your first try, you will receive a gold box; if you do it on your second or third tries, you will get a silver or bronze box, respectively. If you take more than three attempts to complete the level, you will not receive a box.
You can only receive boxes for completing levels in town buildings.
Rewards available in boxes include puzzle pieces, materials to unlock buildings, rubies, boosters, tricks, and time-limited infinite lives. The fewer attempts you require to complete a level, the more valuable your prize will be.