Diamonds are graded starting at D and going to Z before heading into Fancy Colors. D-F are colorless and are more rare and valuable than the near colorless of G-J and so on and so forth.
The graphic above is broken down similarly to how I would explain color, except I tend to make a distinction between G-H and I-J. The average person begins to see color at I, so I break colors into High Near Colorless at G &H and Lower Near Colorless at I & J. That's not to say I or J or bad by any stretch-- the diamonds on my hand fall into this grading and actually go down to K-- but if someone is sensitive to color, I or J will have a tint to them the others won't have.
Color grading diamonds is part science and part opinion. At the GIA Gem Trade Laboratory, it takes at least two gemologists to make a grading call. If the two agree, the grade is given; if the two disagree, a third is called in to break the tie. This gives confidence in a lab report, but if you have your piece appraised by an independent appraiser like me, the accepted margin of error is one grade down or up.
Part of the reason color grading diamonds is subjective is that the color G, for instance, isn't an exact color like Pantone #11-4202. Instead, it's the color between F and H. So, one gemologist may think a diamond is a high G while another would say F. Neither is actually wrong, but the value is impacted. The replacement value of an F is more than a G if all other attributes are the same.
Older diamonds often fall into the Faint Yellow to Very Light Yellow category, so if you see an Old Mine or Old European cut diamond graded at a J, L, or even an R, don't be alarmed. It's par for the course on stones that were mined in the nineteenth century in Brazil and India. It lends to their authenticity and the diamonds are beautiful regardless.
Fancy color diamonds-- diamonds after Z-- are available in almost all colors, some rarer than others, brown is the most common while red is the least. The most famous colored diamond is the Hope Diamond, and we'll cover colored diamonds in another post.
As always, feel free to ask questions, either when you receive your appraisal or now. I am always more than happy to answer them.