Yharnam Stone is a Key Item in Bloodborne.
|A sacred heirloom left by Yharnam, Pthumerian Queen.
"The Queen lies dead, but her horrific consciousness is only asleep, and it stirs in unsettling motions."
- No further use than proving your accomplishment.
- Drops from Yharnam, Blood Queen of the Old Labyrinth
- Stays in your inventory when moving to NG+
This item may also be there to represent the queen's immortality (Much like the Queenly Flesh dropped from Annalise). However unlike Annalise, Yharnam cannot be revived.
One could reasonably assume that this is a so called Lithopedion, a calcified, unborn, dead fetus.