Diamond Guide: The Four Cs Explained

Read about the four Cs (cut, color, clarity and carat) and how they affect the value of a diamond.

Last edited: 10/8/2020
Round white diamonds scattered on a flat bluck surface

Many of your customers will know something about how diamonds are evaluated and graded. They may have heard of the "4 Cs" (cut, color, clarity and carat) but struggle to understand what the phrase means and how it relates to a diamond's worth. Your customers may know what a diamond looks like but not understand the technical terms that are used to describe its anatomy.

Being able to clearly explain how diamonds are evaluated and priced will help you educate your customers, turning them into confident diamond consumers and building their trust in you and in your services.

Diagram of a gemstone


The cut of a diamond is about much more than its shape. How a diamond is cut has to do with how well the facets catch and reflect light. It is this reflection of light that makes a diamond sparkle, which is why cut is such a critical component in evaluating a diamond.

Consider the illustrations below:

Line drawing of a deep cut diamond
Line drawing of a shallow cut diamond
Line drawing of am ideally-cut diamond

If a diamond is cut too deeply, light escapes from the pavilion, and the diamond loses sparkle. Sometimes called "nailheads," these stones are usually unnecessarily heavy (and thus more expensive) and have poor light performance.

If the cut is too shallow, the light passes through the stone without being reflected back to the eye. Often diamonds with a shallow cut appear "spready" and have a large table facet. In shallow-cut diamonds, you can sometimes see a reflection of the girdle through the table. These poorly cut stones are referred to as "fish-eyes."

If the stone is correctly cut, the light bounces back toward the eye through the facets, creating fire, brilliance and scintillation—the characteristics that give a diamond life and make it such a prized gemstone.


GIA color grading scale
*Gemological Institute of America–creators of the International Diamond Grading System

The Gemological Institute of America (GIA) is the preeminent source for diamond knowledge. The GIA diamond color-grading scale is used at all levels of the diamond trade; it ranges from D to Z+, with D being colorless and Z+ being a fancy yellow.

Because it is the absence of color that makes a white diamond valuable, customers who want the highest-quality diamond will look in the D, E, F range. For many customers, near-colorless (G, H, I, J) diamonds are a good option because they are more affordable and still offer a diamond's sought-after colorless properties.

Prices will drop as you go down the scale, with the exception of "fancy" colors.


Because diamonds are formed under intense heat and pressure, it is common to find internal features (called inclusions) in them. Surface features (called blemishes) are even more common.

As with color grading, GIA provides the industry-accepted standard for grading a diamond's clarity.

GIA clarity grading scale
*Gemological Institute of America–creators of the International Diamond Grading System

Flawless (FL) and internally flawless (IF) diamonds are extremely rare and not commonly available. When they do become available, they are very expensive.

Fortunately very, very slightly included (VVS) and very slightly included (VS) stones are more commonly found, and many customers will struggle to see the inclusions—even under magnification. These diamonds are the ideal grade to offer customers who want the finest diamonds that are regularly available.

Diamonds in the slightly included (SI1–SI2) range are also popular among consumers. While they have inclusions that are clearly visible under 10X magnification, they appear clean to the naked eye (and are referred to as "eye-clean"). Diamonds in the imperfect (I1, I2, I3) range have inclusions that are considered "eye-visible" and that may affect the brilliance of the stone.

On the GIA grading scale, there are two SI (or slightly included) grades. Some members of the diamond trade have attempted to high grade their inventory by inventing an unauthorized SI3 grade. Diamond vendors who use this grade are usually trying to sell diamonds that would be more strictly graded as I1 on the authorized GIA scale. At Rio Grande, we never use the term SI3 to describe diamonds. We are strong proponents of faithfully using the GIA clarity grading scale.

Sizing Round Diamonds

Carat Size Normal Range (ct.) Normal Range (mm)
.01 .0071–.012 1.2–1.4
.015 .013–.017 1.5
.02 .0175–.023 1.61–1.75
.03 .026–.035 1.9–1.95
.04 .036–.045 2.0–2.2
.05 .046–.065 2.25–2.4
.07 .066–.085 2.55–2.7
.10 .086–.115 2.85–3.0
.12 .116–.135 3.1–3.2
.15 .14–.17 3.25–3.5
.20 .18–.22 3.6–4.0
.25 .23–.27 3.9–4.2
.33 .30–.35 4.25–4.6
.40 .38–.43 4.5–4.9
.50 .48–.55 4.9–5.3


Not to be confused with karat (a measurement of purity used with gold), a carat is a measurement of weight. One carat (or ct.) is equal to 200 milligrams.

Diamonds less than one carat are sometimes referred to by point size. You can think of a point like a penny. There are 100 points in one carat, and a one point diamond is 0.01 carat.

It is important to emphasize to your customers that a carat is a unit of weight, not of size. Diamonds with the same carat weight can have different size measurements, depending on how they are cut. The chart at right shows the carat ranges Rio Grande uses when sizing diamonds, as well as the equivalent ranges in millimeters, for diamonds up to .50 carats.

Fancy Color

Most diamonds are graded according to the absence of color. But what if color is what you're seeking? Fancy color diamonds are available in a wide range of hues—including yellow, pink, blue and more.

As with colorless diamonds, the rarer a colored diamond is, the more valuable it is. When it comes to colored diamonds, saturation (how rich the color is) also comes into play. Natural, highly saturated colored diamonds tend to be the most valuable.


Diamonds are available in a variety of shapes and faceting patterns. Round, brilliant-cut diamonds are the most common and the best known. Other typical diamond shapes and faceting patterns are illustrated below.

Line drawing of the facets of a round diamond
Line drawing of the facets of an oval diamond
Line drawing of the facets of a marquise diamond
Line drawing of the facets of a pear-shaped diamond
Line drawing of the facets of a heart-shaped diamond
Line drawing of the facets of an octagon-cut diamond
Line drawing of the facets of an asscher diamond
Line drawing of the facets of a radiant-cut diamond


Our GIA-trained gemologists love working closely with customers to find just the right diamond for their needs. You can reach us at 1-800-257-7026 or at [email protected]. We're looking forward to hearing from you!