The Heart of Stone is the heart of the Evil Queen. The witch Hagatha turned it to stone and cut from her body so she would not feel her heartbreak. This way, the Evil Queen can only feel pain or any emotion whatsoever if she touches the stone. The spell literally makes her "heartless".
"A rock in the shape and size of a human heart was on a small table in the corner of the cell."
In A Grimm Warning, Hagatha's sister Hagetta mentions the spell and says it is "dark magic", turning Evly into a "soulless monster". Hagetta used a similar but weaker form of the same magic on Little Bo Peep, cutting out only a small part of her heart and turning it to stone. (see: Little Bo Peep's Necklace)
The Heart of Stone is given to Snow White after the Evil Queen's death.