I go with Frankies Girl for this one. I have a confession to make -- after 40 years of marriage, I spent my own money to buy myself a great big rock. I know it isn't mustachian, but dammit I wanted it for 40 years and I finally got it this past summer. I saved up and paid cash, if that helps my case at all. I bought it "used", but who can tell if a diamond is used or not? It's a 1940's-era platinum mounting with a nice, big, fat emerald-cut diamond in the 2.3 carat range, VS1, I color. It's a little warm. It's a lot of fun to look at. I didn't overpay. I didn't get ripped off in any way unless you consider that I fell for the whole diamond scam. Damn, did I mention it's fun to look at?
I can recommend a couple of things: find out about diamonds. Since I like emerald cut stones, I knew I'd need to stay on the clearer side. A slightly warm color doesn't bother me, so I is fine, but K is rather dishwatery in color. Don't pay for what you can't see. If you can't tell the difference between D and J, then buy J. If you can't see a blemish at SI-2, then buy one of those, why pay for zero flaws if you can't see the ones that are there anyway?
Consider an older or weirder cut. I love emerald-cuts, but lots of people don't. I hate princess cuts and lots of people love them. Lucky me. I love round brilliants, the most common cut, but I love Old European cuts even more. They're thicker, have a smaller flat table on top, and reflect candlelight into millions of beautiful colors. They tend to cost less too.
Get all Princess-Diana and rock a colored gem for your main stone. Her sapphire engagement ring was copied around the world.
I bought my diamond via Craigslist from a family that inherited it and needed to pay for a kid's college. I met the seller in the office of a well-known private appraising jeweler. She let me look at it through the microscope and explained her grading and price. All of this comforted me, even though I know a lot about diamonds now. I paid half what this stone would cost me from a regular jewelry store, and the mounting was thrown in for free. (I love the vintage mounting.)
I have seen lovely diamonds in pawn shops (they use diamond testers and you aren't likely to get a fake stone at least). I've seen awesome ones at really good antique malls, with vintage mountings, cool side stones like sapphires and emeralds, and neat old Euro cuts and even some rose-cuts. Those are wonderful. Get a loup and learn how to use it.
If all else fails, consider a place like Affordable Diamonds Online down in Oregon, there's no sales tax down there and their prices are rather good!
I hope any of this helps.