You're very fortunate to have found love and I'm happy for you that you are engaged. Have you thought about some other means of memorializing the occasion? Despite rationalizations that there are conflict-free or "green" diamonds, they remain the product of an industry fraught with human rights violations of the worst kind. Unfortunately, replacing the diamond with some other precious gemstone doesn't make things much better.
An alternative might be to take the money to set up a fund for a cause that you and your partner both believe in, even if it's only a couple of hundred bucks, and watch it grow together and cash it in for the sake of the benefactor on your 20th, 50th, or whatever anniversary.