If I remember right the example which was seen here came from old clock maker who had two and gave one away.
As about other hones from Snowdonia (please note Snowdonia not Snowdon) there are few already known. Namely DT and YL. With Neils white hone from Snowdon yes it is possible. There are deposits of white mineral. However I am not sure about its honing abilities as to my eye they look like silica veins.
Unfortunately links and photos of the quarry are lost in my old PC. But as for CG from what I have seen so far the quarry is not on Snowdon but on mountain next to it Moel Siabod. It would be helpfull if somebody with proficiency in photografy and access to pictures from the qurry coul give me time of the day and directin they were taken. This would allow to narrow to fewer possible quarry holes.