
Originally Posted by
DaveW
It looks like a carborundum stone of some sort. If it is, it's made of the same general abrasive that is in sandpaper, so you will probably need to use loose grit of some sort to flatten it.
I'd clean it up first by taking it out of the base and immersing it in either kerosene or something warm and soapy if you're averse to using kerosene, and then it'll be easier to tell for sure that it's silicon carbide and not just a dark india stone (it does look more like silicon carbide).
You'll have to determine how coarse it is, it may be too coarse for razors except those with extreme damage - and better for setting bevels on knives and tools.