First of all, congratulations on your upcoming marriage.
I don't want to be a bummer, but am I the only one that thinks 20K for a ring is insane, when you are also saving for a down payment on a home
whether you have the money or not, you're still paying extra for the name. the husband of a good friend of mine and his whole family, run a jewelry business and he's always told me that unless you are in love with the design, not to buy brand name jewelry beacause it's overpriced. and this is especially true for diamonds. you can get the exact same quality for less money. i guess for some people the brand is important as a status symbol and they need that extra validation, and they're willing to pay for it.I don't think the price is outrageous - I replied earlier as DH bought me ring with similar specs from Tiffany when we got engaged. As I said earlier, I think it's an individual decision and none of us know your financial situation. For me, it was one of the best purchases in the wedding.
DH and I had a pricey wedding and live in mid-high COL city... depending on your jobs, $20K is not unreasonable IMHO. The old rule of thumb was 2 months salary I think, and many Tiffany shoppers make that kind of money.