there are jewel molds you could cast a sugar or isomalt "diamond" and then make the band out of GP.
Or you can make it all out of GP and cover the band in gold/silver (the really shiny stuff, not luster) and then on the diamond use disco dust!
When I did mine I used a floral styrofoam piece that was shaped like a O then I cut the bottom half off and covered it in gum paste, let it dry and painted it with silver I used GP for the setting and a large glass stone from hobby lobby as the stone. I just cut a slit in the fondant and slid it down into the cake so it was sitting in a ring box. The box was of course covered in Tiffany blue fondant. White fondant with super white luster dust made the inside look like satin.
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