marble and granite

Bill, the majority of my machining experience has been machining marble and
granite. Marble can vary a great deal in its hardness, and it is important
to note that stone that is often referred to as marble may be limestone or
serpentine. Limestone is softer than marble but more abrasive, serpentine is
as hard as marble or harder. Water is the only coolant you need, but you
need a lot of it. Carbide bits are usually sufficient for marble, for
engraving I prefer a tapered carbide bit. Diamond bits are used for harder
stones like granite. I have only done this type of work on machines
specifically made to machine stone, and frankly, I don't recomend trying to
do it on any other kind of machine. It is very messy, the "soup" that you
get from the water and the slurry is very corrosive (base), and the
approach, or strategy, isn't the same as with metal, etc. If I haven't
talked you out of trying, let me know what the marble looks like and I may
be able give you an idea of its hardness.

Patrick