That would be my guess.
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I did one side, it does not look like there is any layer or band. The side looks much the same as the top.
I hit it with 80 grit on our marble countertop, then 120, followed by 400 wet. It looks flat now I want to finish up with 600 grit. I'll get a photo later, the color is a much lighter shade of brown now, I wonder if thats just old darkened oil? I will get the stone out of the wooden box and start to soak. Have not seen "Green" around here..... would regular dish soap work?
Use "DAWN" or something like that if you have it, it is designed to break down oil and grease.
Arkies work best with oil, so there's not really a need to clean them. I would just use it as is for knives.