Whether you’re buying a diamond engagement ring, anniversary diamond stud earrings, or just a special gift of diamond bijoux for your lady — the key is to be an informed shopper. Do your research first and you will be rewarded by her thanks and a lighter tap to your wallet.

Shape (Cut)

If you don’t want to come out directly and ask your girlfriend which shape diamond she prefers, you might have to resort to asking her friends, sister, or mom. I can’t really help you with this since this is purely a matter of taste; no matter what I tell you about the most popular shapes, it won’t matter unless your sweetheart genuinely likes what you buy her.

On the other hand, if you still want to take a chance in order to keep her in suspense, your safest bet is probably the classic round brilliant shape. This is, hands-down, the most popular shape simply due to its traditional look. If you’re concerned about getting this wrong, make sure to check the return policy on the diamond before you purchase it — usually there is a 30 day window.

Carat

Admittedly, this aspect of the diamond’s appearance is what grabs people first. You need to be concerned about the carat weight of the diamond you are purchasing for two important reasons. First, don’t disappoint your woman. What will happen if you buy her a half-carat diamond and she is expecting at least 1.5 carats? And second, the carat weight of the diamond, more than any other single factor, will have the greatest influence on the price tag of any given diamond. Be sure to do your research on how many carats you are expected to gift but keep in mind the general rule of diamond carats — the bigger, the better.

Cut Quality

Okay, now that you’ve figured out the answers to the more aesthetic questions of shape and carat, let’s focus on cut. The tricky thing about the diamond’s cut is that it’s not standardized across the board like shape and carat weight. Different vendors assess cut differently. The bottom line here is that you should take into consideration the cut grade and proportions of the diamond’s certificate, for example from the AGS or GIA lab, over the individual diamond retailer.

Also, if you are looking for a round brilliant, stick to ideal/excellent cut. But if you are looking for any other shape, also called a “fancy” shape, then widen your search to premium cut or disregard the cut criterion altogether.

Color

The diamond's color may not be as readily noticeable as the carat but it still matters greatly in terms of the price of the loose diamond. Diamond color is assessed based on a letter scale with D being perfect, i.e., “completely colorless,” and the farther down the alphabet you go, the more yellow the diamond. But don’t pay for something you can’t appreciate! Your goal should be to get a diamond that looks colorless once set even if it’s color grade is not perfect.

Though there is much to say on this topic, I generally take two points into consideration when advising people about diamond color. The first is the color of the metal in which you choose to set the diamond. It’s important to know if you are setting the diamond in white gold/platinum or yellow gold because the color of the metal will affect how you see the color of the diamond. Generally, when you set a diamond in yellow gold, you can get away with a lower colored diamond than if you use white gold or platinum. And the other point is the diamond’s shape.

As a rule of thumb, there are three categories of shapes with regard to color:

(1) round brilliant

(2) Asscher, princess, and emerald, and

(3) all other shapes.

Depending on which of these shapes you choose, you will be more or less restricted in the color of your diamond.

Clarity

With clarity, your goal should be to get a diamond that is, what we call in the industry, “eye-clean” — that means you can’t see any imperfections (known as inclusions) with your naked eye. If you go above and beyond that, you are paying for something you can’t appreciate. You are not going to walk around with the diamond certificate to show everyone that you bought your wife a flawless (IF-FL) diamond. Save your money on getting a better carat size or color.

A diamond’s clarity is assessed using the following scale. At one end of the spectrum is IF-FL, which stands for “internally flawless-flawless.” Then comes VVS1 (very, very slightly included – first degree) and VVS2 (very, very slightly included – second degree). Next is VS1 and VS2, and then  SI1 and SI2. The last category is I1, which means the diamond is obviously included and you should stay away from it. As with color, your decision about the diamond’s clarity will be greatly affected by which diamond shape you choose.

Go Brilliant

So get out there and do your homework! Find your woman a brilliant diamond that strikes her fancy, and save yourself some money from being an informed shopper. Whatever the happy occasion, just remember, diamonds are a girl’s best friend. ;)

Ira Weissman + is the founder of Truth About Diamonds , the premier site online for diamond consumer education.