For god's sake, don't go to a mall. I spent a lot of time researching this for the fiancee's ring. Pricescope is a good place to start, as well as diamondtalk.com for info.
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If you are really interested in a big diamond, you can get a "shallow cut", which is much wider than ideal so that it looks bigger when set in a ring. It does not reflect light well, though, so it may look more like a piece of glass than a diamond. Another option is a princess cut diamond. These are generally cheaper than round cuts, so you could probably get more carat-weight for your $$. I personally decided that I should only pay for the features of the diamond that I can see with the naked eye. That's why it wasn't important to me to have a flawless diamond; I just wanted one that had no flaws that are visible with the naked eye. As far as color goes, after looking at rings for a while you can easily tell the difference between a D and an I, or even an F and an I. I's look like crystallized urine in my opinion.
Cut quality is the most important factor. This determines how much light is reflected out of the top of the diamond among other things. Better cut diamonds cost much more, of course. You could get a huge poorly cut diamond for $10k.
Another thing to consider is whether or not you want a certified diamond. A certified diamond has been graded by an independent lab, so you can be more sure about what you're buying. I would never buy anything but a certified diamond, especially over the internet. There is a large variability between the labs, though. IGI, EGL, and several other labs that you are likely to hear about in mall jewelery stores have very loose standards. A certified diamond from these labs is barely better than a non-certified diamond. The best labs are GIA and AGS. Certification adds value to the diamond.
Word of advice: do not believe anything that a mall jewelery store tells you. In general, any non-certified diamond they call a D is F or G at best, and what they call an H is at best a K or an L. Buy from a reputable non-mall jeweler, either online or locally. I would recommend buying the diamond online and getting it set locally.
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