Steve,

From another thread, you asked if it's possible to rotary engrave granite.

Although I haven't engraved granite, I have engraved green marble, by using diamond burnishers.

The results were acceptable, although, since marble has various densities throughout the piece, some areas engraved fairly deep, while other areas barely engraved.

I think granite would have a more consistent density.

The engraving itself comes out with a contrasting light color, but Rub 'N Buff can be used to add color.

If the material is too porous, Rub 'N Buff will get into the surface, in areas where you won't be able to remove it. :S