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Silicon Carbide - most dime-a-dozen old stones that aren't good for much but utility or early stage knife or chisel sharpening are made of silicon carbide (I.E. Crystolon) or aluminum oxide (I.E....

8" x 2" x 1" doesn't really mean much. Many good razor stones were historically available in that size. Silkstones, Arkansas stones, Scotch hones etc. The photos don't look like a SiC stone to me.