I have a piece of one of these stones. I suspect it's part of the shoulder of the stone and has been through a few fire/quench cycles. The original face is smooth and rounded, the inner sharp-edged and angular. I can bring it out for a walk - say to Leazes Park - if necessary. It's been registered as a portable find; I picked it from a stream bed, and there is a curious feature to it - at the termination of the groove.