This is a very enlarged pic of the surface of a Llyn Idwal hone:



Very like a CF but not quite as fine a finish. Colour is a drab grey/green (the one in the pic has been wetted and looks darker than it is) and has very faint bands of speckles in it.

Perhaps yours is one of these? I take it that it took a long time and was very hard to lap - the stone should be glassy hard, not like the coticule, thuringian or naniwas that you have.

Regards,
Neil