I have the advantage of having a jeweler who is a distributor, so he gave me more knowledge than I will ever need about the diamond industry. I've posted this before, and some people have found it helpful.

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Find the Rapaport online, it will give you the best idea of what a diamond should cost. It is the industry standard for pricing and updates quarterly I believe. It isn't meant to be public, but there are certain sites out there who will have a copy, even if it is an old one.
Personally, I buy diamonds from a distributor, above a 'wholesaler', 'dealer' or individual store. Diamonds, like any item go up in price the further they travel. The best bet is finding a distributor who gets the diamonds straight from New York (point of entry for all diamonds). You will likely have to pay in cash or in full before you can take the finished product.
Avoid online jewelers like Blue Nile. They loosely group their diamonds into far more broad ranges than a jeweler would. They buy diamonds in bulk that are all 'similar' to the type that they are selling, they could be anywhere from 2-4 steps off of what you think you are buying. You could end up paying $1000 for a diamond that was supposed to be a certain weight, color and clarity but they send you a diamond that was close enough to the untrainned eye but ends up being worth $800 when appraised.
Know what you are looking at, learn the 4 C's before you go, this will also help with the Rapaport. Personally, buying a loose diamond and having it set is far more personal than buying a completed ring that anyone can buy. Always ask to use the jewelers magnifier to examine the diamond yourself. A good jeweler will point out the flaws in the diamond under the glass for you, this is so you can realize why the diamond was rated the way it was.
For metals, if you are going with a white/silver colored metal, I would recomend paladium over platinum and white gold. Paladium is a precious metal that is slightly more expensive than white gold but far less than platinum. White gold is essentially bleached gold and will yellow over time and need to be redone to return to its 'white' color. Platinum is far more expensive but also far more labor intensive, it scratches far too easy and must be buffed out periodically. Paladium is brighter, more scratch reistant and whiter than both the other two. For the price, I don't think you can beat the look.
Have an idea what type of ring she would like: solitare vs. three stone, plain band vs. design, square cut vs. round cut, etc. If you have a basic idea, you should be able to customize a ring that she will love.
Best of to him luck with the search.