I was doing a friction fire session with some archaeologists last Friday, and one of them asked me about this.
Having seen my limpet shell set up, and the old, worn wooden bearing block that I also had he mentioned finding largeish lumps of chalk with conical wear patterns in them. The conversation came about as I mentioned that all you would find in the ground of my kit would be the shell or a stone with a worn dimple in it.
My initial thought was that it would be too soft but as I haven't tried it I don't know for sure.
Any comments from anyone?