The style of ring my girl wants would be a small fortune at tiffany's and I think you paying mainly for the name there.
No you wouldn't be paying a small fortune, but think about what paying for the name Tiffany & Co and the Blue Box means:
Tiffany is celebrating 130 years of introducing the engagement ring and you'll get superior quality to any of these Zales, Jared, Helzberg Diamond type companies where you think you're getting a good deal, but you're actually getting low quality diamonds or diamonds that aren't cut at a high level or have a lot of inclusions or have chips on the facets of the diamond
Facts:
-Tiffany only carries diamonds with a color range of D-I, so even if you got an I, you would be leaving with no worse than a near colorless diamond
-Tiffany grades on Presence, which measures a diamond's brilliance, scintillation and dispersion (Basically how light comes through the diamond and the sparkle you see from a diamond)
-Tiffany diamonds are hand cut, hand crafted and hand polished and have been doing this since 1886
-Tiffany rejects 99.96% of all gem grade diamonds...you're getting the best possible diamond at Tiffany
-You'll also get a lifetime warranty from Tiffany for your purchase as well
Don't waste your time spending money on bullshyt diamonds, especially if you really want to spend that amount of money. Tiffany doesn't add chemicals or colors to its diamonds like other companies do to try and enhance it. If you're serious about your fiancée and about this purchase you owe it to yourself to at least do the research and comparison shop, but I would strongly recommend you go to Tiffany and ask the questions and see the difference for yourself