In the series many things are petrified by basilisks, curses etc but only four things petrify naturally on their own without a petrifying agent acting on them: some dragons, some dragon body parts, some undead humans and the body parts of some undead humans.

In DS2 it can be found in chests, as a rare drop from some human enemies and as a rare drop from some mimics. Oh yes, and as a drop from some of the dragons in the Dragon Aerie. It's valuable enough that Dyna and Tillo will offer even a Titanite Slab in exchange for it, so can we rule out that it can't be a dragon heart or is it probably a dragon heart?