Apparently multiple items can give you white hearts, like the ones given by Fate.
What are these white hearts? What do they do, and what can give them to you?
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White hearts (which the wiki calls Eternal Hearts) sit above regular hearts, but below soul hearts in the order of what takes damage first. When you fill up a white heart or you go down a level with half a white heart, it turns into a heart container.
According to the mechanics guide, any regular heart drop has a 1/50 chance of becoming an eternal heart. This excludes hearts from the Bloody Penny or Little C.H.A.D., which are always half hearts. In addition, in the super secret room that contains a single eternal heart, any other regular heart drop in that room automatically becomes an eternal heart.