[Dick Linn]

The milling cutters you get with your mill may be quite expensive. Someone may get upset if you ruin one trying to bull on through a hard magnet just to see what happens.

I wouldn't worry so much about the color, as the magnet might not have been ground after sintering. Looks like many of the Samarium Cobalt magnets are black, and still extremely hard.

Grinding or machining anything containing Iron with a diamond isn't a great idea. Iron has a strong affinity for carbon, which it will gather from the diamond. Grinding steel on a diamond wheel is a good way to ruin it.