I don’t know if this helps or not, but I have a couple pair of stones that have never been used, 50/80 diamond, and 1200/1600 ceramic. When I hold them together, up to the light, I can see a little slice of light shining through the middle. Yet, when I hold a straight edge to each of the sides, they appear very flat, and no light shines through. So, perhaps when they are held together, there is some anomaly? I don’t know… Just something I noticed.