Carat is the measure of how much a diamond weigh. Carat is also related to size (how big a diamond looks), although it’s not the only factor that determines size. Depending on the shape, weight distribution and cut quality, two diamonds with the same carat weight can be of different sizes. It’s worth noting that some diamonds are cut solely with an emphasis on weight. These gems can sacrifice brilliance and symmetry in favor of delivering a larger carat figure.

Caratage is the easiest of the 4 Cs to understand and measure, inasmuch as it refers to weight. Furthermore, because the density of diamonds is essentially constant, as the caratage of a diamond increases, so does its relative size. It is simple to obtain, because all that is required to weigh the diamond on a scale. Unlike the other three Cs – color, clarity and cut – it is not subject to human interpretation.

The term “carat” originates from the Greek word “karaton,” which was the carob seed that was used to balance scales in ancient times. A carat is equal to 200 milligrams.

It is important to distinguish between “carat,” which refers to the weight of a gemstone, and “karat,” which refers to a fineness of a precious metal.

Carats usually are expressed a full number or zero to two decimal points, such as 2.15 carats 0.60 carats. Hundredths of a carat are sometimes referred to as points, meaning that 100 points equals 1.00 carats.

Another term that is sometimes used is “grainer.” It dates back to the trade in the Far East, where diamonds were weighed against grains of rice, with 1.00-carat equaling four grains of rice. Members of the trade will today refer to four grainers (1.00 carat), six grainers (1.50 carats) and eight grainers (2.00 carats). A one grainer could alternatively be referred to as 25 pointer or a 0.25-carat diamond.

The price of a diamond does not necessarily rise at rate that is consistent with the rise in number of carats. Larger diamonds are rarer, so they typically have a greater monetary value per carat. In other words, the value rises at a faster rate as it gets bigger in size. And there are other factors at play. For example, a 0.95-carat stone is 5 percent smaller than 1.00-carat stone, but it is more than 5 percent lower in value. Passing the 1.00-carat benchmark tends to boost a stone’s value, because of market demand. This does not exist between a 1.01-carat diamond and 1.02-carat diamonds, but it does between a 0.99-carat diamond and a 1.00-carat diamonds.

Understanding what diamond color means helps in choosing the right diamond. Interestingly, the diamond color evaluation of most gem-quality diamonds is based on the absence of color. A chemically pure and structurally perfect diamond has no hue, like a drop of pure water, and consequently, a higher value. D to Z diamond color grading system measures the degree of colorlessness by comparing a stone under controlled lighting and precise viewing conditions to master stones of established color value. Many of these diamond color distinctions are so subtle that they are invisible to the untrained eye; however, these distinctions make a very big difference in diamond quality and price.

The majority of natural diamonds fall within a color range that runs from colorless (sometimes called white) to near-colorless, and then to lightly yellowish or brownish.

The slight variations that occur in a diamond’s color form the basis of a generally accepted color grading system, first introduced by the Gemological Institute of America (GIA), which uses the letters of the alphabet. In this system D is the highest grade, and it is assigned to a completely colorless or white diamond – a rare phenomenon.

Near-colorless diamonds are graded with the letters E and F. The most common color grades encountered by diamond consumers run from the color grade G through the color grade M. Diamonds that display a slight yellowish or brownish hue receive the color grades K, L or M. The color grades N, O, P, Q and R represent stones with a progressively light yellowish tint, while the grades S down to Z represent diamond that show an increasingly yellowish or brownish hue.

When diamonds are of a truly vivid color, for instance a lively “canary” yellow or a distinctive pink, they are defined as Fancy Colored Diamonds. Here, the alphabetical color scale does not apply and they generally are described according to their hue, and the intensity of the color.

Clarity refers to a diamond’s natural inclusions. While small marks within a diamond are natural, their appearance can leave something to be desired if they are visible to the unaided eye. The shape of a diamond can affects the importance of its clarity grade. The facet patterns of the brilliant-cut diamond shapes such as round and princess can hide certain imperfections, but step-cut shapes such as emerald and Asscher have large, open tables that make inclusions more obvious.

Clarity refers to the purity of the diamonds, or to the degree to which it is free of blemishes and imperfections. In principal, as the clarity of the diamond increases, its value is enhanced.

The clarity of a polished diamond is affected by both external blemishes and internal imperfections or inclusions, some of which were created in nature when the diamond was formed and others during the cutting and polishing process. Internal inclusions include piqués, or dark spots, gas bubbles or lines, and even tiny crystals. Sometimes cracks in the diamond crystal, or fractures or feathers affect clarity, passing through the interior and sometimes reaching the surface.

Clarity characteristics are what make each diamond unique, since there are no two stones that will have the same exact inclusions in the same locations. Evaluating diamond clarity involves determining the number, size, relief, nature, and position of these characteristics, as well as how they affect the overall appearance of the stone.

The following are the commonly used gemological terms to define clarity:

FL, IF (flawless, internally flawless): A grade given to a polished diamond where no external blemishes or internal inclusions are visible using a standard 10X-magnification loupe. Experienced jewelers seldom see such stones, and some even argue that flawless diamonds simply do not exist.

VVS1, VVS2 (very very slightly included): With VVS1 being the better of the two, such grades are assigned to diamonds in which imperfections and inclusions are very difficult to spot, even under the magnification of a 10X loupe.

VS1 and VS2 (very slightly included): With VS1 being the better of the two, such diamonds appear clean when seen with the naked eye, but the inclusions are relatively easily spotted with a 10X loupe.

SI1, SI2 (slightly included): With SI1 being the better of the two, these diamond’s inclusions can be detected with difficulty with the naked eye, but are quite obvious under the magnification of a 10X loupe. SI3, which is used by certain labs, although not GIA, is a grade slightly lower than SI2.

I1, I2 and I3 (included): With I1 the best of the three, these grades are assigned to diamonds where the inclusions can be seen easily with the naked eye.

The term diamond cut refers to the quality of a diamond’s proportions and symmetry. These elements determine how much light a diamond captures and reflects, and therefore how much it sparkles. This is a direct result of the skill of the craftsman who shaped and cut the diamond. There are Five grades of diamond cut: Ideal, Excellent, Very Good, Good, and Poor. If you compare two diamonds of different cuts grades, you will see that the higher cut grade has significantly more sparkle. There will also be a difference in their prices, with the better cut grade commanding a higher price.

The quality of the cut, or the “make,” is crucial. It is this that decides how the light entering the polished diamond through its table – the largest area, facing upward – will react, and consequently how brilliant the stone will appear. A poorly cut diamond will actually loose light and appear dull, and its value will be significantly lower than a diamond with of the same size, color and clarity, but of an excellent cut.

Diamond dealers sometimes use the word “cut” to refer to the shape of the diamond, but in reference to the 4 Cs it specifically refer to the physical factors influencing the interplay of light within and from the polished diamond.

These, according to research carried out by the Gemological Institute of America (GIA) decide the stone’s brightness, which refers to the internal and external white light reflected from the stone; fire, which is the scattering of white light into all the colors of the rainbow; and scintillation, which quite simply is the amount of sparkle a diamond produces, and refers to the pattern of light and dark areas caused by reflections within the gemstone.

To determine the cut grade of a diamond, GIA considers the proportions of those facets that influence the diamond’s table-up appearance. It also takes into account the design and craftsmanship, and the quality of polish on the facets. GIA provides an overall cut grade for standard round brilliant cut diamonds in the D-to-Z color range, with the grade appearing as either: Excellent, Very Good, Good, Fair, or Poor.

Currently GIA does not provide an overall cut grade for non-standard round brilliant diamonds or fancy shaped diamonds. But its reports do contain an assessment of the diamond’s polish and symmetry; a description of its shape and cutting style; its measurements; and a simplified proportion diagram containing a description of girdle thickness.