Can you be more specific? What is the goal of the photograph?
Are you trying to make a “pretty picture” or are you trying to
depict some feature like an inclusion or girdle inscription? Each
would dictate an entirely different approach.
For the latter:
www.thelittlecameras.com with Diamond
Dealer’s Kit perhaps.
Another solution is to use a microscope with the attachment
necessary to attach your camera of choice (point-and-shoot or D-SLR).
You can also obtain a small tubular USB camera that fits on a
microscope so you can show your customers the features of the stone
on your computer or on a TV.