What is this abrasive 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

it looks a lot like silicon carbide, as would be seen on a 'dressing stick' for cleaning\truing a grinding wheel. aluminum oxide would be finer grain, and more homogenous in white, blue, pink, green.

hope that helps.

Answer 2

It appears to be either lava rock or pumice