The Piggy Bank is a regular in-game offer that gives you the chance to buy extra diamonds for real money at a good price. If you want to take advantage of this offer, you need to beat levels—each win adds 70 diamonds to your piggy bank.

As soon as you have 600 diamonds in your piggy bank, you can buy it for real money and get the diamonds, or you can continue saving them up. The maximum amount of diamonds you can save in your piggy bank is capped at 1,000.

Keep in mind that the Piggy Bank offer doesn't affect the number of rewards you normally get—each victory will earn you just as many prizes as it always does. This offer is simply another opportunity to buy diamonds at a fantastic price.

You can fill your piggy bank and open it more than once before the offer ends. If there are any diamonds still left in your piggy bank when the special offer is over, you'll be able to start from the same amount the next time the special offer comes up.