Get one of these. I absolutely LOVE mine....
The Epicurean Edge: Japanese and European professional chefs knives
I'm tired of eating up diamond plates lapping stones. This thing makes extremely short work flatting ANY stone. It does leave a fairly coarse surface but just use another flat stone that's fine to smooth out the surface. So... flatten your finishing stone and flatten something else coarser, like a 1000 grit or so. Then use it to smooth out the roughness left on your finishing stone. Works like a charm. I have never had a problem with "stone contamination". Read the fine print with the GDLP... it says not to use it on anything coarser than 10 grit. This thing will work on anything and last near forever.