Buying a diamond (real or manmade) is not an investment where you expect it to appreciate in value and eventually be worth more then you paid for it. Just try to sell your ring to a jewelry store and you will find that are worthy virtually nothing. The only ongoing value is in the gold/silver/platinum the band is made from. You mostly buy diamond rings/jewelry to wear/enjoy, not because you think you are going to make a profit from it.
I agree as others have mentioned, the monopoly around mined diamonds has been built up over the years and is partly clever marketing. Particularly for a couple just starting out in life, they probably have way better things to spend their money on then a mined diamond. No one you show it to will know any difference and there is no need to tell anyone which type of diamond it is.