We went with the Asscher cut. Simple, elegant and a little different from the usual.
Used BlueNile.com. Very, very pleased with them. Wide selection of diamonds, you can view the certificates online, nice collection of mountings. Wendy liked sitting in her living room and choosing from thousands of stones. You can add a handful of stones to your basket and a bluenile rep will call you to discuss the ups and downs of each one and how they can address any downsides with mounting options.
Some people will urge you to one company or another, or to the "wholesale district" to get the best deal. Frankly almost every stone today is part of a big wholesale pool and every company...even the "wholesale district" culls from the same pool. I saw the same stones (verified by their posted certificates) on many wholesaler and retailers sites, all at different price points. Blueniles were right in there on price.
Quality of cut in my opinion is the most crucial element. That will affect the brilliance and light reflectiveness. A big high quality stone with a bad cut looks like crud.
We mounted the stone (~2.17 carat) in a gold setting, so frankly color didnt bother me much since the stone was going to pick up all sorts of yellow from the mounting. I think it was an "H" color. Mounted in platinum or white gold...you might consider something north of the G color. "Blueness" in a stone refers to a slight blue tone. Years ago blueness was highly sought after and people paid extra for it. Now its considered a flaw. Its almost impossible to detect up to moderate blueness without a black light, and some blueness will cancel out some of the yellow color in a medium colored stone since blue is opposite yellow on the color wheel. So if you're not a modern day purist, a JKLM stone with small to moderate blueness will look to the naked eye like a FGHI stone. And in 20 years blue may be the in thing again. High blueness makes the stone look oily and murky though.
Clarity wise its a VS2 stone...better than pedestrian but far from perfect. There is one single surface inclusion that a bluenile customer service rep called to tell me could be easily covered by one of the prongs on the setting. I paid for an appraisal at a local jewelry store and they didnt even note the inclusion, appraising it for a little less than 2x what I paid. My insurance company appraised it for a few grand more than that.
Now dont get me started on how debeers used some exceptional marketing skills to completely create the whole diamond engagement ring business (100 years ago a ring was not common and when one was offered it was usually a family heirloom and almost never a diamond), how they control diamond prices, how the stones are far from rare, impossible to sell used for anything better than 10c on the dollar, or the labor conditions that they're mined with.
Kinda funny story...when my uncle got married to my moms sister back in the sixties, she wanted a huge diamond ring. He offered her a choice...furniture for their new home or the ring. She took the ring. They sat on lawn chairs and used cardboard boxes for tables for the next 2 years before he finally relented and bought the furniture.
I'm guessing he didnt see her naked very often for that two years.