My advice is to get to the diamond district and cut out one layer of the transaction. Also, I feel it should go without needing to be said but do your research and negotiate! If possible do the purchase in cash and you can save a lot if you have planned thoughtfully.
For the record -1 emerald at 1.26 carats vs1 and two trapezoid .88 set in a custom platinum setting (completely off of Tiffany's website). Grand total $12,500 for the ring and engagement weekend away.
This ain't Brewster's Millions - don't be foolish by going to Tiffany's, even if your father/grandfather has the money.