I think it is scripted to take it...
I tried multiple ways to steal it off, even with a high sneak (over 5) your other characters other than the talking one usually get stunned till the conversation is over.
Also in one of my saves, before the initiation of the conversation if I recall corectly, I abused a low fps drop so one of my characters could grab the stone , but they imediately got stunned or struck in conversation can't recall, but i do recall the bloodstone popped from me like i would manually throw something on the ground and unto the conduit hehe, it was like , gimme that it's mine and i was like woooot?! xD