I haven't read this entire thread yet, but my first suggestion would be to join Pricescope.com. There you'll find lots of information on diamonds and on getting the best value when purchasing diamonds.
I've never put pressure on men to buy me expensive things. I would've loved any ring my fiancee picked out for me. He ultimately asked for my help picking out a stone and setting, since I know something about gems and jewelry. I offered to split the cost of the ring with him (he refused), and then offered to buy him a ring for himself (he accepted).
I liked the early suggestion one poster made of asking your fiancee to buy you something equivalent in price. This is, after all, 2015. Or you can ask her to cover half the cost of her two carat diamond. Neither of these may go over well, but if she considers the act of plunking down $12,500 of her hard-earned cash for one-half of a ring, she may begin to see the benefit of smaller stones, or vintage non-diamonds.
Good rubies, sapphires, emeralds, tanzanites, alexandrites (they change color, too!), and certain types of demantoid garnets are among the rarest of stones. Vintage rings set with these gems would be a great replacement, but only if your fiancee would really like a different gem. White diamonds aren't rare; they're actually the most common gem. They've simply benefited from a long and effective marketing (and hoarding) campaign by De Beers.
http://www.gemsociety.org/article/are-diamonds-really-rare/If she's still determined to have a large diamond, I'd consider cut quality the most important aspect. Without an ideal cut, the stone won't be as lively as possible. If you get an ideal cut (you can read up on how to review a stone's cut measurements), you may be able to purchase a 1.5 carat stone with better presence than a standard or very well cut 2 ct. I'd also suggest going with AGS or GIA rated diamonds. Since not all cuts rated "excellent" or "ideal" are equal, review the diamond measurements themselves (and, obviously, how the stones present in person) to determine the best cut for you.
http://www.pricescope.com/tools/hcaAs you already know, don't go to a retail jewelry store (Tiffany, Cartier, etc). They don't give out enough information on the stones they sell, and their markups are insane. De Beers has already ensured the price of diamonds will probably never be equivalent to their real value, so I personally wouldn't allow a retail store to add to the already-crazy diamond markup.
