Just trying to think around the problem here. But, in the past, i was given a stone, by @joamo , and it had some cracks that were toxic. I filled them in, with a good epoxy. This did the trick. Is there a way you could use a Dremel Tool, with a cutting disc, remove the toxic line out, maybe 1/32 inches deep??? Just a very narrow channel, and fill it in with epoxy?? If you want, you could use the dust from the Koppa, and lapping stone, to mix with the epoxy, to give it a little bite. ..