I assume you are asking about making DeBeers or Tiffany's quality images. Their photographers may still be shooting 8x10 film as far as I know.
Possibly.
This is about the one thing I have not done (that and nudes) but as with anything that is simultaneuously highly reflective and refractive the lighting will be all about highlight placement -making sure the light goes only where you want it. One approch that apparently works well is to work with very small light sources : BAlcar and a couple of other companies make light modifiers that sit like caps on a conventional flash head and pipe ight through fiber optic cables, if you don't have access to those ou might consider pointing your lights at small mirrors rather than the stone itself and use those to aim the light to exactly the spots you want.
That will also increase the apparent distance between stone and light source. That is key. A diamond has a very high dispersion, and it's faceted to take advantage of this. So, the facets have dramatic color changes with small changes of the angle of the light. A two degree shift in angle is enough to change the reflection from orange to blue, so a light source that covers two degrees (as seen from the stone's point of view) will cause that facet to reflect both orange and blue (and all the colors in between), which then recombine to make white.
Basically, the light sources want to be less than a degree as seen from the stone. So, the end of a 1/4 inch fiber optic should be at least 15 inches from the stone if you want each facet to show a strong color. Cut the distance in half to make the color more pastel, which is more like the way most ads show it.
A brilliant cut stone will typically only show about 1/4 the facets bright and colorful with one point source of light, so you just keep adding sources and playing, so that each new source adds new facets without double or triple lighting too many existing facets (which will cause them to lose their colors and go white).
Make use of - -as you suggested -- black cards and even black tape to precisely tune the shape and size of the light , control reflections on the facet surfaces. Once you have "woken up" the stone then think about carefully adding an overall diffuse light from the top of the set.
Did anyone ever tell you that you have a gift. You're explanation of how you guess you'd shoot something that you don't have experience with is pretty much bang on. And despite not having done it, you're explaining it better than people who have done it could.
You should take up writing
Small spotlights maybe a better choice than electronic flash as you'll be abse to see in real time what you are doing with light.
But not LED spots. They have spectral peaks that show up as weird bands in the reflections from a stone.
Along the same line of thinking: If you can work with your camera tethered to your computer so you are not making critical judgment calls about lighting based on a tiny screen on the back of your camera or the peephole of your viewfinder, and instead see it at or close to full resolution on a computer monitor.
And of course, shoot raw to make the most of your camera's dynamic range, particularly up in the highlights where you will need to be able to separate outsubtle differences in tonal gradations and retain detail without the jamming that JPEG captures will do.
I've been known to composite three shots. One with an insane amount of small, intense lights which is basically so that the stone throws colored patterns onto the foreground. One more "correct" for the specular reflections, and a final one correct for the diffuse lighting.
My God, I used to do that on a 4x5, with masks, and grease pencil drawings on the ground glass.
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Rahon Klavanian 1912-2008.
Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.
Ciao! Joseph
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