Piggy Bank is an additional offer in the game. It allows you to accumulate gems and get them at a special price. 
You can save gems by:
- cutting any object in the game
- completing Amanda's orders
- completing Franco's orders

Please note that you receive 1 gem for cutting out the objects and Amanda's orders, and 2 gems for each Franco's order.

You can open the Piggy Bank after saving at least 60 gems. As soon as you open it, you get the next one, which should be filled with gems from scratch.