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They 'grey' rock is a finer grit than the yellow one so I think SC is out of the question.

If it is SC what is the probable grit of the stone? I can't see any online that shows over 240grit??

Would love to see a similar stone.

the grey stone is pretty uniform throughout, sanding on the side revealed the same surface as the top of the stone so I doubt it's natural.

If it is silicon carbide would it be close to something...

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Picked these up at a local garage sale in Nova Scotia. These cost $0.50 each so even if they are junk i'm ok with it!

The 'greyish' stone had a deep dish in it and ate 2 belts of 80 grit...