Welcome to CakeCentral...
warning this site is addictive!
so here's a diagram of the basic shape.
reading left to right....
far left ....profile from side
center...looking straight down
right top -- profile of diamond area looking at it from the front to show how carved at angles and undercut to simulate shape of diamond
bottom right -- how you could carve it out of a 9x13 cake....
carving the ring area so it is open wastes tons and tons of cake.
use the pattern to carve shape of the diamond ... scale to look nice in comparison to a large round cake that you use whole to make the ring.
make gold ring of icing or fondant around the outer part of the round cake and make the center black.
Put message on black area.