Best Cutter/Bit for Engraving Black Onyx - Expert Advice

Question
hi Steve,
my name is Manny, i have a small engraving company and recently i was approach by one of my costumers to engrave an initial on a black onyx oval shape. what kind of cutter or bit do i need to put on my engraving machine to cut through the top surface and come out clean, the head rotates like a milling machine.
please tell me what i can do, this could be very big  quantity for me with this costumer, maybe you make the bit or you know some one who does, pleas respond.
thank you
Manny

Answer
HI Manny,

I would diamond bits.  Maybe a sphere shaped head.  I would need to know what the chuck can handle for size of the shank of these diamond tools.  These would be probably one eight inch size.  But you can get smaller and larger.  The sphere size can be different sizes also.

Regards,

Steven D.Covey