Ah, right! They did use objects for hearts. I got the idea from an anime I'm pretty fond of, Escaflowne, where large gems were what you'd procure from a dragon's heart and it was used as a power source.
But heartcores in this verse have several shapes/sizes, we ~TECHNICALLY~ have seen Suko's already and it was fairly big.
wasn't it the same way in sailor moon? various shapes colours and sizes but always some form of jewel? i remember the villain removing Tuxedo masks jewel and it being huge and apparently powerful... i can't find much online though since this show is so friggin old
That and they had mirrors (in SuperS if I recall right, to view their dreams), swords (Uranus' pure heart crystal if I recall right), ornate bells(came from the two heart talismans), and icicle looking crystals (called "pure hearts" in the dub) and such as well.