Ok, so i know I'm a little late on the game here, but maybe it'll help another guy reading this.
A little personal background - I worked in jewelry for 7 years and have worked at both mom&pops and the BIG boys (and I mean BIG). I work with my FH as his receptionist at his clinic now so please don't feel I'm trying to sway you to one store type or another. The mom&pops were way more fun to work at, the big boys payed the rent. C'est la vi! I figure if I can't use all my expensive education at work anymore I'll just get my fix here!
Ok, now to you question. Yes, buying a diamond is like hacking your way through a jungle for most guys. There's a ton of "advice" out there and it's hard to know what's true and what's a sales pitch. Plus, you have a whole new vocabulary to learn since most of us don't grow up learning diamond 101 in school.
What I usually suggested to my grooms to be was to sit down and figure out your budget first. It's the least glamourous part of the process but it needs to be done. Set a figure that YOU'RE comfortable with. It should be something that won't leave you in a cardboard box, but at the same time you want to be proud of what you're giving her. (Now is not the time to be a cheapass

) The good news is, that besides your future wife, this is one of the few things about the whole wedding process that maintains it's value pretty well! Once you've set your budget decisions will become easier. Keep in mind when you're shopping that you have a number. You want to spend less than that number and the salesperson wants you to spend more (it's how they feed their families). Budging 5-10% may not kill you but if someone gets pushy showing you something you're not comfortable with just walk away. Believe it or not there are a LOT of options out there.
Next, you need to prioritize. As posted above some people want big, others want perfect. It truely depends on you and your FW and there is no right or wrong answer. Number these things in importance to you:
Cut - 1st the shape, Round, Princess (square), Oval, Marquise (football shaped) etc. There are a lot to pick from but the most popular are round and princess. Once you have the shape decided you should inspect the proportions, symmetry and finish. This is where you need a professional. It's darn near impossible to truely know unless you've had the training. In the long run though what matters is - How sparkly is it to your eye? And not in the bright lights - hold it away from the showcase. When they're clean and new they all look great, but once she's worn it for a week or two if it's cut is poor even the slightest amount of dirt will make it look dull and it will loose its "life". Most cut grades on the market are good/good. That's the average. They start poor,fair,good,excellent. You will also see a lot of specialty cuts out there for example: Peerless, LEO, Hearts Desire, Ideal, Millenium etc. Every company has their own. They each have their good and bad sides but the one thing they all have in common is a higher price tag. Keep in mind that more facets does not always mean better - in fact, some studies indicate that the human eye can't recognize more than the standard 58 facets. With a little leg work you will probably be able to find a diamond that looks as good, but isn't as pricey, as the name brands. As you see, I've spent a lot of time on cut. That's because it accounts for appx. 60% of the price of your diamond.
Color - Color starts at "D". There is no ABC and anyone using those grades is making up their own system (be wary). There's a good story for why it starts at D, but this is long enough. The breakdown goes like this:
DEF - colorless
G-J - near colorless
K-M - light yellow
You won't see much out there beyond that - they turn into industrial diamonds
Color is a bit tricky. Girls tend to have a sharper color eye than boys so a feminine perspective may be helpful. Most people are happy D-G, however an I with an awesome cut can hold it's own. Keep in mind that you will pay a higher premium for that F and so a G can be a great way to cut a few dollars without sacrificing the look.
Clarity - Clarity refers to the lack of inclusions in a diamond, aka things that either are not diamond or are a structural irregularity in the stone. The grading system looks like this:
F/IF - Flawless / Internally Flawless - VERY rare, not usually seen at the average store. No inclusions visible under 10x magnification.
VVS1/VVS2 - Very,Very slightly included - Minute inclusions visible at 10x
VS1/VS2 - Very slightly included - Minor inclusions visible at 10x
SI1/SI2 - Slightly included - Noticable inclusions visible at 10x
I1/I2/I3 - Included - Obvious inclusions visible at 10x
Most of what you see on the market is VS-I. The 1/2 denotes very slight differences in grade at most levels. The exception is at I. An I2/I3 typically has a surface reaching inclusion that can compromise the integrity of the diamond. Surface reaching inclusions can chip, crack, even split your diamond in 2 if hit just the right way. True, they have gone through the intensive process of being cut and mined, however it only takes the right strike for it to crumble. Something as simple as doing laundry (I've seen it happen).
It is important to look at the diamond under magnification for clarity characteristics. A gemscope is best, but a loupe will do. Just make sure you do a little research online and practice using one. Nothing screams "sucker" more than a guy who is looking through a loupe completely wrong. Trust me, you will get made fun of.
Carat - Easy, "How big is it?!" Everybody's favorite question. Besides being no one's business it is another big price factor. How important is it to you. Would a large diamond make your FW blush and feel inappropriate? Or does she want everyone in space to be able to see it? This is completely personal. Some pointers though, there are a 100 "points" in a carat, so theoretically you could get a .95ct diamond and no one would notice the size difference from a 1ct. This really only works well at mom&pop store. The bigger stores buy in bulk and sku their items. At a larger store there may have been a nicer, bigger, diamond purchased in that lot that is priced the same as the "average" quality of the lot. The other thing to consider is the mounting. This can make a big difference in how big the diamond looks. If you are looking at loose diamonds make sure to look at several mountings to get a feel for this.
Some other things to consider:
Return/exchange policy - just in case
certification - make sure you're getting what they say you are
reputation of the company - are they going to be there for you if a problem arises
warranties - some of the big stores have cool plans that cover your diamond/ring sizing etc for life, just make sure you read the fine print
appraisals - go to a 3rd party appraiser for the most accurate
insurance - check out
www.jewelersmutual.com
In the end, just remember, when you find the right diamond you'll feel right about it. What matters is the look on her face when you ask her to spend the rest of her life with you. On a side note - do some research on cool proposal stories and really put some thought into it. This is a moment she'll always remember
Sorry, I got a little involved and this turned out really, really long. Sadly, I could probably keep going. If you have any questions feel free to e-mail me. bartelstobe at hotmail