Hold the box and stone in your hand and take a small hammer tap one end of the box down with the hammer. Chances are it is not glued to the box except by the slurry.
In effect you are tapping the box away from the stone. Once it starts to move, rotate the stone and tap the other end. Keep switching ends until it is free.
I have done many stones this way. It does not take much force. I use a 2 or 4oz peen hammer.
Nice find.