I can confirm Nelly's earrings are diamonds. Seen them many times... 10 carats a piece and very nice quality. They're probably the biggest solitaires that exist in the hip-hop/athlete jewelry game today. In my experience I've seen and sold lots of athletes 5-7cts each, but Nelly may have the title for this one. Jeezy's are also real but appear to be something like 5cts each.
To chime in on the moiss talk... At least 99% of fake hip hop jewelry is CZ. Moissanites are very rarely used in hip-hop jewelry, there might just be a handful of moissanite hip-hop jewelry pieces out there period.
If you're in the market for an engagement ring, get her a diamond. You can get a nice quality starter ring for $2-3k and upgrade from there as needed. If you buy a moissanite you won't feel good about it, and she won't either. Also a secret... Lots of women go straight to a jewelry store after they are proposed to and find out what their ring is worth, so don't hide anything because she'll find out quick.
-z