i just made a similar cake today for pickup tomorrow. the ring is made with gumpaste, cut a stripe as wide and as thick as you would like and drape it over a cake pan allowing the ends the hang. for the diamond, you could mold it out of gumpaste and paint, make a mold and use melted sugar or buy a glass like paperweight from hobby lobby and mount that in the middle. The ring will take several days to dry hard, then paint it with nu-silver or gold lustre dust. HTH
thank you so much!
How did your cake come out?