You're gonna have to get more specific on what "decent" means to you (and her).
It amazes me how in fine jewelry you can look at two seemingly identical pieces, but one costs $1k and the other $10k.
So what does decent mean?
That said, here's what I found close to your budget in a couple minutes of procrastination. You can shave a lot of coin off the price of a ring -- to a point. No chance you're getting anywhere near 1.5ct at a $2k budget. That said, you can spend wisely and get some nice bang for your buck. What I've selected here in a round cut diamond at 0.7ct that clocks in at pretty darn good on all fronts. It won't win a gemology contest, but she and her friends will all ooh and aah over it and be none the wiser. No sense paying thousands of extra for crap you can't see without a microscope, unless you have thousands extra to burn.
Now here is the kicker: You can use the setting to buy you a lot of mileage from a low cost stone. Here I've selected a halo-style setting, essentially using a lot of small cheaper diamond to surround your primary stone and give the overall impression of something much larger. By surrounding the main stone with smaller ones, it also makes any imperfections in the center stone less visible.
Total cost $2745 before taxes.
Build Your Own Ring - Review | Blue Nile
The only question is whether or not she would like the style, and for that you're on your own!
Not sure if she just likes what she likes or what, maybe learning more she can get away from spendy pre-done store rings. Building your own can save money but also get you something you like better.
4 C's of diamond appraisal:
Color
Clarity
Carat
Cut
It was Blue Nile, or the internet, that taught me the highest color and clarity that is practical, where beyond that, you can't tell the difference practically speaking.
The cut can make a difference on those points, as well as setting.
Put this by her, have her play on the sites. You can print something from Blue Nile that shows the approximate carot size for the various cuts.
Princess and round cut look "brightest" and have a rainbow effect. I think round cut is cheapest of the 2. They hide flaws in the diamond better, one reason you see them so much. The store can get a higher profit margin on these. You also "lose" less weight on the starting diamond cutting these.
Emerald cut, my fave, has a more vintage look, doesn't look as "bright", but it "draws the eye deeper into the diamond." It also "throws more dramatic flashes of light" off when moved about. It takes a larger starting size diamond to be cut compared to others, and to be clearer and whiter. However it still tends to be slightly cheaper than the other cuts because it's not as popular (more unique among friends!)
I can't remember the last most common cut.
It can really help price when you know just what you want for diamond and setting.
I'll let the sites help you compare settings.
If you go white gold it is cheaper than platinum, but will need to be refinished at some point (it's regular gold that's been plated with silver for color).
Every metal but titanium and platinum are mixed with some amount of nickel, and could be a problem with metal allergy, at any point in the future.
