I don't think it is an extremely rare stone, more like just rare. It looks like novaculite, if the white lines on the upper side of the first picture are small cracks, like flaking. I don't think UK has nay other novaculite hones accept from Charnely forest and Llyn idwal. Llyn idwal is hard like glass usually, charnley is softer but still hard. Llyn idwal is a bit coarser than Charnley, there are stones near the 8k grit. After that, I'm not sure.