The four C’s of gem grading refer to color, clarity, cut and carat. Gemologists, jewelers and other experts grade gemstones based on these four properties. It is important to understand that the grade of a gemstone in essence reflects the rarity of the features of the gem. In practice, not every use case demands that the highest graded gems are also the most suitable ones. Additionally, the influence of personal taste and preference is just as important when selecting a gem.

Color is actually a rather broad term that is not convenient to describe the properties of a gem in fine detail. Therefore, in gem grading color consists of multiple properties that all contribute to the overall color of the gem. Three of these properties are hue, tone and saturation.

Hue: Hue refers to the dominant color of a specific color family. Hues can be thought of as the primary and secondary colors of the color wheel. These colors would commonly be described as yellow, orange red, violet, blue and green. White, black and gray are never referred to as hues.

Tone: Tone refers to the relative lightness or darkness of the gem. As a reference to painting, paint tones include a pure hue mixed with a certain amount of gray. By adding gray to the pure hue, it becomes less intense.

Saturation: Saturation refers to the intensity of the hue. The color of a gem can be stronger or more soft depending on the level of saturation. For example, orange becomes a shade of brown as its saturation increases, whereas pink actually is desaturated red.

For colored gemstones, the gems with the highest value generally are the stones with purest hues and strongest saturation. Especially for high-valued gems, subtle changes in color can make a huge impact on the value of the stone. Also, the specific color properties of a gem can affect the clarity of the gem making it easier or harder to spot inclusions with the naked eye.

Clarity grading refers to the evaluation of inclusions and impurities that impact the visual and structural aspects of a gem. Inclusions refer to any material that became trapped inside the gem while it was being formed or created. These inclusions will interfere with the passage of light through the stone.

Magnification plays an important role in clarity grading as not all inclusions are visible to the naked eye. In some cases, the term “eye-clean” might pop-up. Eye-clean means that a gem has no inclusions or blemishes visible to the naked eye, but under magnification the gem still might have one or more inclusions. The difficulty of spotting inclusions depends strongly on the type of gem, gem color and the cut that has been used. 

Two other important factors influencing the clarity of a gem are fractures and veils. Fractures and veils are serious inclusions that tend to weaken the gem. Gems with fractures tend to be more prone to breaking as they impact the structural integrity of the material. Gems should always be assessed from all angles as inclusions can be more or less prominent depending on the viewing angle. In addition, the type and quality of the cut also impacts the ease with which certain inclusions can be seen.

Cut refers to the facets, dimensions and ratios that as a result of the expertise of lapidary artisan make up the eventual shape and cut of the gem. For natural gems, from all of the four C’s, the cut is the property that artisans generally can control the most. Also, cut is one of the harder aspects to judge and requires serious training and experience as the cut quality is influenced by many factors. Some of these factors are symmetry, magnification, facet junctions and windowing.

Symmetry

Unless a gem is intended to be “freeform”, it is usually the intention for most gems to have a regular shape. For these regular shapes, symmetry plays an important role in the quality of the cut. The stone should be symmetrical in the right directions and not bulge. Again, symmetry should also be checked from all sides and angles to be sure the cut is highly symmetrical no matter how it is viewed.

Magnification

The cut of a gem should always be examined under magnification. Under magnification the facets of the gem should have a smooth, mirror-like appearance. Pits, scratches or dull areas are an indication of a poor cut or polish.

Facet Junctions

For faceted gems, all facet junctions should align in the correct angles as the junctions should have a crips look and come together in a single point. The further the facet junctions are off or rounded, the more the brilliance of the stone is lost.

Windowing

Windowing refers to the situation in which light passes straight through the gem rather than reflecting back. Avoidable windowing tends to occur when the cut has a large face with a shallow bottom. When a gem has an obvious window, the area in the centre of the stone will show reduced color while the edges actually look darker.

Carat is a unit used for measuring or describing the weight of a gem. In most gem descriptions, the term carat is abbreviated as “ct”. Carat should not be confused with the term karat. Karat is a term used to describe the purity or fineness of gold. Karat is abbreviated as ‘K’.

The weight of gems is often expressed in fractures (e.g. ½ carat) or in decimals (e.g. 0,5 carat). A carat can be divided further into 100 points. Generally speaking, larger stones are more rare than smaller ones. Therefore, larger and heavier stones tend to have a higher price value per carat. The unit weight of carats is expressed as:

1 Carat = 200 milligrams (mg)

5 Carat = 1 gram (g)

5000 Carats = 1 kilogram (kg)

1 Carat = 100 points

0.01 carat = 1 points

In jewelry, when a higher carat stone is not suitable or available for a given application, the technique of clustering is sometimes used as an alternative. Clustering is a technique used to give a stronger appearance to a set of smaller gemstones. In clustering, multiple smaller gemstones are set closely together in such a way it becomes challenging to tell them apart. Because of the small distance between the stones, the combined appearance gives a stronger impression that resembles a higher carat gem.

The four C’s of gem grading stand for color, clarity, cut and carat. The grading of gems indicates the rarity of the features of an individual gem. Depending on the intended end-use, the highest grade may or may not be the most suitable choice. In addition, personal taste and preference are also decisive factors for picking the perfect gem.

Color

The color of a gem consists of multiple different elements. Some of these elements are:

• Hue, refers to the dominant color of a color family.
• Tone, refers to the relative darkness of a gem.
• Saturation, refers to the intensity of a hue.

Clarity

Clarity refers to the evaluation of inclusions and blemishes in and on a gem. Inclusions interfere with light passing through the gem and therefore affect its appearance. Structural inclusions such as fractures and veils tend to weaken the integrity of the stone.

Cut

The cut of a gem refers to the facets, dimensions and ratios that an artisan creates on a gem. The overall quality of the cut is influenced by many factors. These factors include symmetry, facet junctions and windowing.

Carat

Carat is a unit used to indicate the weight of a gem and is abbreviated to “ct”. One carat equals 200 milligrams and in turn one carat consists of 100 points. Carat should not be confused with karat (abbreviated as “K”), which is the term for describing the purity of gold.