Diamond in the rough and a mystery stone

[HNSB]

I bought an old coticule on Ebay. It had been used with oil, so it was hard to tell what the underlying coticule looked like, but there was no doubt from the pictures that it was a coticule. I set a snipe and forgot about it...
Well, I ended up winning and when it arrived yesterday I started to clean off the oil and lap it. I was very pleasantly surprised to see a light creamy colored coticule underneath all of that oil.
I attached an in progress picture, and a comparison to some of my other coticules. (it's very hard for me to get colors right in the pictures, especially off of my phone)
In the pic to compare to other cotis, the small one on the left is my personal favorite for it's speed. Then is the new one, and then a couple of other grayer ones that I have.
This new one does have some cracks in it, but I don't feel them when passing a razor over, so I'm hoping they'll be ok.

Then, later in the day I stopped at an antique store and picked up a few razors and the mystery stone. It was also covered in oil, but the smoothness and color of the surface (which had a pretty thick coat of oil on it) made me think it might be a thuringian. Well, when I got it home and started cleaning and lapping, it obviously is not a thuringian. It reminds me of a carborundum, but it is much finer than any I have ever seen. Based on scratches under a microscope, I'd say it's just over 1k in grit... But, it cuts FAST. I could not believe how easy it was to set bevels with this stone.
I've attached a picture, I wonder if anyone has any ideas...