If you can feel it with your nail, there is a problem, and follow any of the above steps. If not, keep using it as if there was no crack. Those situations are pretty common, especially on natural stones. So, if you can feel it, then, there is a problem. If not, it's more of an aesthetic defect, that doesn't necessarily affect honing. Also, try honing on the stone like there was no crack. If you see an actual difference in performance, then, there might be a problem. If not, then, keep using it, and, just don't look at the crack. Looks can be deceiving, and nortons are not the most beautiful stones that exist anyway. So, hone, and, then tell us what you think.