Two facts:
1) Russia is a big source of diamonds and they are in serious need of hard money right now, so they're selling diamonds by the truckload to buy food and fuel.
2) Sri Lanka is a big source of diamonds and, while our media haven't given it much coverage, that poor country is in the middle of a terrible civil war. The rebels control most of the diamond mines, and they're selling diamonds by the truckload to buy guns and ammo.
The result is that now is a poor time to sell a diamond.
IMHO, a 1/2C diamond is to much for the average guy to give to a woman he's not at least engaged to. My advice is to set your windfall aside and save it for an engagement ring when you're ready.
You're mother will find that a mature decision, I'm sure. I'd suggest explaining it openly and honestly to your girl friend. Just say, "I've decided to save it for use in an engagement ring. I hope to give that ring to you someday, but I'm not ready to do that right now."
Chuck