The Core Breaker was updated in 1.5.6.
All cores have been organized into 4 new rarity categories and can be opened in sets of 10.
Any cores you had before 1.5.6 are now under these new groupings: Common, Uncommon, Rare and Fountain. http://pbs.twimg.com/media/ExrxHxeWgAATyeX.jpg