I think the simplest solution would be to simply ask the co-worker if he would like the box that came with the stone that he bought from you, then order a new one. He/she obviously needs a stone, and took it upon himself to use yours without asking. I'd say he bought it and simply has not paid you yet;)
I borrow tools from guys all the time...it goes without saying that if a tool gets damaged, the borrower replaces it.
It would hurt to loose something with high sentimental value, but I would not take something like that to work...