The idea of an engagement ring with a large diamond was heavily marketed by De Beers diamonds. This was when it was more common to lose your virginity during an engagement or after the wedding, as opposed to after 4 or 5 Smirnoff Blacks.
The larger diamond obviously equates to more money spent, which would supposedly prove the mans desire to stay with her and not just blow and go. In effect, it was a bid on her virginity.
I hope you enjoy all the symbolisms embodied in this gesture. Not least the fact you are acknowledging that a woman's vagina has a price tag, a man's worth is judged by his credit rating and that taking out a $10k+ loan on something that can be accidentally swallowed is a sensible financial decision.
It makes all the children who died digging up that rock worth it for the pawnshop who will end up with the ring in 2-8 years.