Thankfully, it's NOT ruined. Thanks John and others for your helpful suggestions. It was indeed just gunk from the Chinese 12K, big relief.

I had some Barkeeper's Friend and a stiff brush, and started digging out the gunk. It actually worked better using just my fingers and the damp powder, than with the brush. I knew jwoods was right when after a good bit of scrubbing, the nice white Barkeeper's Friend powder started turning the very familiar dark "Chinese 12K Gray."

I tested it afterwards with Bart's glass test suggestion, and it cuts just great. Can't wait to try a razor on it.

I've already touched up a few already-sharp razors with the Chinese 12K, and think it's great, especially for the price. It's a nice slow polisher that really smooths an edge after a lot of strokes, giving me time to learn basic honing technique. Pretty much what I was after.