The chick should be marrying you for the person that you are. Not for jewelry, cars, and insurance policies. A ring out of a Cracker Jack box should do the trick.
That being said, if you've been basing the relationship on a lie.... that you have a lot of accumulated wealth and buying a real, $10,000 diamond ring is no problem when it actually is and you're thinking of faking her out with a lab-grown diamond... then it sounds like you have a much younger model whom is sure to leave you when she finds out the truth and it all comes crashing down.
So.....two ways to go here. First one is to be honest with yourself and her and let the chips fall where they may. The second one is to give her a lab grown diamond ring, acting as if it's a $10,000 ring, with no intention of marrying her, and have fun while it lasts.