Today it’s hard to imagine buying diamonds without the 4Cs, the handy mnemonic device GIA created to describe diamond quality, and the familiar diamond color grades. But the diamond color scale is a relatively recent invention created in the 1950s by the Gemological Institute of America.

The GIA color scale ranges from D, for completely colorless, to Z, for diamonds that are light yellow or brown. (Fancy colored diamonds, which have more noticeable color than Z diamonds, have their own separate GIA color grading scale that describes their hue as well as the intensity of the color.) When you think about transparent white diamonds, think about a continuous color range that starts at colorless, the complete absence of color, increasing in color seamlessly to diamonds with tints of yellow or brown.

Why do the GIA color grades start at D instead of A like every other grading scale you’ve ever used? GIA wanted to make sure that its diamond quality grades were not misused or confused with informal diamond color grading scales that dealers used at the time.

Diamond color grading in a laboratory is a methodical process with tightly controlled conditions. It starts with the environment. Diamond color is graded in a light box with a specific light bulb with a known spectrum that’s equivalent to north indirect daylight. Loose unset diamonds are graded upside down against a standard white background.

The most important principle of diamond color grading is that it’s always done by comparison using diamond color master sets: diamonds that represent each diamond color grade. That’s because even for professional graders the tiny differences between diamond color grades can really only be seen consistently in comparison to other diamonds of known grades.

Color in diamond is a little bit like blending custom paint. You start out with white diamonds with a complete absence of color and each color grade has one drop more of color blended into it. You may not easily see the difference of each drop but when you look at the beginning color and the color with many drops it’s easy to see the difference.

You only see the faint hint of color in a G or H color diamond by putting it next to an E or F color diamond. If we show you a fine quality colorless diamond in our office or even outside in the sunshine you won’t be able to tell what grade it is. Most people can’t even discriminate between three grades unless the diamond is upside down and next to a diamond that’s a grade or two higher or lower.

Obviously, you won’t be wearing your diamond loose and upside down next to a master set. So the premium price awarded to diamonds of the highest GIA color grades is more about rarity than beauty. And the distinction will only be recognizable on your diamond grading report, not by looking at your diamond in an engagement ring.

Anyone in the diamond business will tell you that they’ve seen diamonds that have received the wrong color grade or diamonds that sit in between two grades that can go either way.

Different labs have slightly different standards and procedures for drawing the line between the color ranges of each grade. At RockHer, we only consider color grades on GIA diamond grading reports: we want to compare apples to apples as much as possible. And GIA is the world’s most recognized grading laboratory, internationally regarded as the benchmark. Diamonds graded by less reputable labs will trade at a discount because of uncertainty over the color grade.

Since most people can’t see individual diamond color grades, we generally talk about diamond color grades in groups or color ranges. Choosing a group, rather than an individual diamond color grade makes more sense because diamond color is a natural color continuum, there aren’t discrete intervals. Where does G end and H begin? It may be different in different labs on different days. Graders are human too and a grading report is only an opinion, even at the GIA.

The first group of diamond color grading scale is the colorless group: D, E and F. These diamonds are the most rare and the most valuable. To accentuate the lack of color in these diamonds, we recommend a platinum engagement ring.

The next group is near colorless diamonds: G, H, I, and J. These diamonds will look colorless in mountings. (You may start to see a slight hint of yellow starting in I color diamonds, especially when set in white metals.) We think that this group offers the best combination of beauty and value: you have a colorless gem but you aren’t paying for the rarity of the colorless group that isn’t perceptible in normal everyday wear. We prefer G and H diamonds, which we think look colorless even in white metals. But if you are setting your diamond in a yellow gold setting or engagement ring, the slight yellow cast of an I or J diamond probably won’t be noticed.

The third group is diamonds with faint color: K, L, and M. We think these diamonds have an off-white color that you will notice in your engagement ring. We don’t recommend this group and don’t offer them on our website.

Not every diamond of the same color grade looks alike. Since color grades are assigned when the diamond is face down, they don’t take into account the optical effects created by a diamond’s brilliance and light performance.

Diamond shapes and diamond cuts definitely do affect the way you perceive diamond color. Round brilliant, cushion-cut and princess cut diamonds, especially well-cut ones that display hearts and arrows, will look brighter and therefore whiter.

Step-cuts like emerald cuts and asscher cuts show slight tints more easily because they have bigger facets. If you prefer these shapes, you may want to choose a higher color grade if you are looking for a white diamond.

Fancy shapes with points, like marquise and pear shapes, may show more noticeable color at their points. You may want a higher color grade than you would a round brilliant.

In general, the larger the diamond, the more you will notice faint colors since the facets are larger and tinted areas are easier to see. So for a larger diamond to appear colorless, you may want a higher color grade.