Sorry, looked at the picture several times, but too difficult I suppose?
Though I could try a guess. Probably you don’t mean the stone, but the penlike object behind it. It could be a pencil, refill, because of the dark tip. But in that case that tip wouldn’t be that straight in form? Such a nice thing with wooden shaft and shining brass. Could the tip be of metal too? In that case it could be for drawing on masters for stencils, or something alike? Etching perhaps?