Hearts of Ice are items used in the Treasure Hunter minigame. They can freeze the different prize categories of the minigame to prevent those rewards from being earned. For example, if you have already reached level 99 in Farming, you may not want Farming related items, so you could freeze the farming category.
Hearts of Ice are used each time a treasure chest is opened, so if you have frozen 4 categories, 4 hearts will be used every time you open a treasure chest.
Note that items from frozen categories can still be placed into a chest, but if won, a new item will be chosen for you from an unfrozen category.
To freeze a category, click the 'categories' button in the top-right corner of the Treasure Hunter interface. You will then be presented with the interface shown to the right. Click on the icon for the category you would like to freeze, and you will no longer receive items from that category. Click the category again to unfreeze it. You can only freeze 10 categories at a time.
Hearts of Ice are included as bonus items when purchasing keys from the billing page. They can also be earned for completing quests, daily challenges and redeeming key tokens and double keys tokens. You will also earn 5 Hearts of Ice for opening 5 chests in one day - there is no limit on how many you can earn through this method.
You are also given some Hearts of Ice for free each day. Members get 2 free hearts rather than just 1. Unlike daily keys, Hearts of Ice do not expire if they haven't been used.