Purple Ruby is a quest item in the Blood and Wine expansion.
It can be found next to a skeleton (presumably George Herbert's) and George Herbert's letter, just outside the entrance to Termes Palace Ruins's underground, on the northern end of the ruins. It's needed to remove the barrier to continue looking for the diagrams in the underground cavern.
This item can be removed from its place by the barrier after it has been put there, even after the quest has ended, allowing to close the barrier once again. There is an interesting dependency on whether it is left in its place or not: when the barrier is left open, wolves and bears in the area around the ruins are replaced by higher-level barghests, most of them appearing near the Termes Palace Ruins signpost and a few along the road leading north to the drawbridge. Closing the barrier despawns the barghests and the natural wildlife reappers.