Alan, A few tricks for photographing diamonds/jewelry:
1) Pull back. Find out what your maximum focusing distance is. This
will give you the sharpest image. People generally push their macros
far to close to the subject.
2) Use a tripod.
3) Use a shutter release cable (if you can), in our case we set the
shutter delay at 5 seconds or so, and this gives the camera a chance
to settle down after pressing the shutter release.
4) If you have a white hold balance on your camera you can light
with even a halogen lamp and get an excellent result.
5) Use a TIFF format.
6) Adjusting color in printing your images is also possible in the
computer, ie color balance, contrast, etc.
7) We've made a small quansit hut as a platform -- a sheet of white
velum arched over the top to diffuse the light and a sheet of white
paper as a sweep. We don't have any 'hot spots'.
8) We use small blocks covered in aluminum foil or white tape to
accentuate the reflections.
Hope this is of some help. Cathy and Tom