It seems that most jewelry sites have almost the same diamond education information out there, verbatim. There is a difference between providing technical information versus explaining how to interpret and use the information practically when shopping for diamonds. Here is an abbreviated primer, with my notes based on helping customers select the right diamond.
Cut: I would say that this is the most important C, since the optical brilliance of the diamond is tied to its proportions and angles. The top flat facet of the diamond, called the table, should be a certain percentage relative to the diameter of the diamond. The depth percentage (specifically the depth of the lower part of the diamond) should also fall into narrow tolerances, along with specific angles of the crown and pavilion. In addition to these percentages and angles (called Proportion), the quality of the polish (how smooth the surface of the facets are), and the symmetry of the facets are also part of the overall cut characteristics. Unlike the other C’s, there are multiple ways to grade cut. GIA (Gemological Institute of America)grades a diamond in the following terminology: Excellent, Very Good, Good, Fair, and Poor. AGS (American Gem Society)grades cut using numbers 0-10, with 0 representing an Ideal grade. You will also see terms like near-Ideal, Fine Cut, etc. These terms don’t mean a lot, and can vary in their accuracy depending on who is doing the grading. You can actually find out a diamond’s potential cut grade if you know a few of its specific measurements through GIA’s Facetware Cut Grade Estimator. Note: These grades and proportions apply to Round diamonds. If you are looking at another shape, they are evaluated a little differently. More guidance on that in another blog.
Bottom Line: Understanding the Cut grade is critical to the value of the diamond. Ideal and Excellent cut grades from grading labs such as GIA and AGS translate to beautiful diamonds with incredible sparkle. They will also cost more. Even a diamond that is E color and VVS clarity will not have beautiful brilliance unless it is cut very well. If you can splurge on just one C, I recommend going with as fine a cut grade as possible. Especially for an engagement ring, this is important.
Color: Color in diamond terminology refers to the relative absence of color in the stone. If a diamond is absolutely colorless, it will reflect the most pure spectrum of colors. The diamond color scale begins at D (complete absence of body color) and goes to Z. Colors D, E, and F are considered colorless, with minute differences only visible if the diamond is turned on its side and viewed against a neutral background. G, H, I and J are considered near-colorless. G, H, and possibly I all look quite white in a face-up, mounted position. J can start to have a slight tint of color to it when in the mounting, but it’s really a case-by-case judgement.
Bottom Line: When choosing a color for a diamond, D, E, and F are fabulous, but they can also be costly due to their rarity. If you have the budget and want to prioritize, you can’t go wrong with a colorless diamond. If you want to balance the other factors (cut, clarity, and carat), especially in an engagement ring, you could consider a G or H–both are spectacular when mounted. If you are less sensitive to color (some people just can’t see the subtlety), an I or J could work great, especially in other diamond jewelry like earrings, bracelets, and pendants. Color grades K through Z would be suited for fashion items.
Clarity: Diamonds are graded on their relative freedom of inclusions. Inclusions are small characteristics that are within the diamond, such as pinpoints or tiny crystals of carbon, and feathery fractures. Most of the time, these are incredibly minute, only visible with the aid of a 10x power microscope.
Diamonds with a Flawless (F) or Internally Flawless (IF) grade have no internal characteristics visible under 10x, and are incredibly rare. VVS1 and VVS2 clarity grades are also rare, and it is very difficult for an expert to see the internal characteristics under 10x. Inclusions within VS1 and VS2 clarity grade diamonds are still quite difficult to see under 10x magnification. SI1 and SI2 clarity diamonds have inclusions that are somewhat easy to see under 10x, but can rarely be seen with the unaided eye. Included diamonds, graded I1, I2 and I3, have inclusions numerous and noticeable enough that you can see them somewhat easily with the eye.
Bottom Line: Clarity is one diamond factor where you can have some flexibility. You can choose diamonds from IF to SI2, and you probably won’t be able to see any inclusions with the eye. That gives you a lot of room to play with, and can be a way to control the ultimate price you pay for a diamond. Personally, I think that Vs2 to SI1 would be a gorgeous, no apologies clarity for an engagement ring or more important piece of jewelry. If I wanted to maximize size, or it is for a piece of jewelry like an earring or bracelet, SI2 would be a great option as well. If you are not concerned about inclusions or just can’t see the difference, you can get a larger stone for the money by going with I1. I don’t recommend I2 or I3: the beauty is just too compromised, and there are durability issues.
Carat: This C is the most objective of all the factors; it is simply how much the diamond weighs. Diamonds are valuable, so they are weighed very precisely on a metric scale to the hundredth of a carat. One carat is divided into 100 parts, called points. A diamond that weighs .49 ct is described as 49 points, or almost 1/2 carat. Diamonds are routinely cut into standard fraction sizes of 1/4, 1/3, 1/2, 3/4, 1, etc.
Bottom Line: Small differences in carat weight can translate to large differences in value. Sometimes you can maximize value by going with a carat weight just under the standard fraction sizes (a .90 ct versus 1.00 ct). Really, you will just have to balance carat weight with other quality factors. You will never regret higher quality, so if you have to choose color and cut over size, that’s a good way to go. If size does matter (who are we kidding, of course it does!), then perhaps choosing a larger, well-cut diamond in the H-I color, SI2 clarity range might just get you what you want.