Hi, for the diamond what I did was to put a little of blue into the white fondant and marble it. Though, just a tiny bit, and baby blue, so it´s barely noticeable. And then add luster dust to the where the spark of the diamond would be, to mark the different angles I hand painted them in a light gray.
For the ring, the first time I did it, I colored the fondant grey and then added luster dust, now what I do is, airbrush in silver or gold, for the diamond you can also airbrush details.
Good luck, and I hope this helps.
This is one of my favorite cakes http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1281321 you could PM eneq, maybe she can help you.
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