Photography is my hobby so I shoot alot of shots with my DSLR.
But unless you have a very good macro lens, your cell phone might take better pictures than a DSLR with regular lens because cell phones can focus with very close range.
Set up is most important. First, you want a either white or black background (A4 paper will work)
Then with either your night stand or lamp, shine at extremely close proximity. It's better if you have more than one light source.
Be creative, have fun, and above all; remember that diamonds are geometric sculptures of mirrors. If you place a dark camera near a diamond, your diamond is going to look dark, so be aware of the diamond's surroundings and place things in the frame to overcome them.
Good luck!
I'm not a great photographer but sunk is correct. I take the best pics with my cell phone in bright sunlight. I keep a white cardboard envelope under the seat in my car just for spontaneous photo shoots of my rings.