What is the abrasive grinding stone in the photo?

Question

http://img392.imageshack.us/my.php?image=abrasivestoneww8.jpg

I have this small (half inch) piece of abrasive stone that scratches glass. I use it to cut very thin microscope coverglasses. Do you know what may be the material? (silicon carbide, aluminum oxide, boron carbide or something else?)

Answer 1

Looks like a granite to me with a lot of pyroxene (which would scratch glass)