Corundum is very hard, diamond will scratch it. Nothing else. If you have a file and you cannot scratch your specimen, then that is possible.
Just a few more questions. Are all three of these pictures the same rock?
If they are, the difference in color of the two opposing sides may be from minerals from the area which have stained it.
If that darker color rock was corundum, i do not believe that shelf would be cut into it in the second picture.