The Four Cs relate to the four categories whose relative values determine the quality and value of a polished diamond. They are carat, clarity, colour and cut. A diamond grading certificate provides an accurate report about the diamond’s Four Cs.

Carat

The weight of a diamond is expressed in carats (in contrast to the weight of gold, which is expressed in “karats”). The word “carat” is derived from “keration,” which is the Greek name for the pod of the carob tree, whose individual seeds weigh approximately 0.2 grams. In Roman times in the gem trading city of Byzance, carob seeds were used as units of weight for pearls and gemstones, but it was only in 1907 that the weight of the carat was fixed according to the metric unit 0.2 grams. 

Carats are subdivided into points and there are 100 points in one carat. Another term for weight that is commonly used in the trade is the grainer, which is a quarter of a carat.

Clarity

Pure diamonds are extremely rare, and almost all contain impurities or “inclusions.” These are traces of non-diamond materials that were trapped in the mineral during the stone’s formation. Typically, many inclusions are removed during the cleaving, sawing, cutting and polishing of a diamond.

Internal inclusions include piqués, or dark spots, gas bubbles or lines, and even tiny crystals. Clouds are clumps of microscopic inclusions that lower the transparency of the diamond crystal. Sometimes cracks in the diamond crystal, or fractures or feathers affect clarity, passing through the interior and sometimes reaching the surface.

 
The following symbols are commonly used to define clarity in a polished diamond:
• LC, FL, IF (Loupe Clean, Flawless, Internally Flawless)
This is the grade given to a polished diamond where no external blemishes or internal inclusions are visible using a standard 10X-magnification loupe.
• 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.
• P1, P2 and P3/I1, I2 and I3 (Piqué/Included)
With P1 or I1 the best of the three, followed by P2 or I2, these grades are assigned to diamonds where the piques or inclusions can be seen easily with the naked eye.

Colour

Most diamonds are referred to as “colourless,” which in the diamond trade ranges from absolutely colourless to yellowish. A small percentage of diamonds display vivid colours, including yellows, reds, pinks, green, browns and black. These are referred to as “fancy colours.” 

The standard colour grading system for diamonds in the colourless range that uses the letters of the alphabet, in which D is the highest grade. D is assigned to a completely colourless or white diamond. Near-colourless diamonds are graded with the letters E through I. Diamonds displaying a slight yellowish or brownish hue receive the colour grades K, L or M.

The colour 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.

Cut

The quality of a polished diamond’s cut, or the “make,” as diamond dealers often say, decides how the light entering the stone will react, and consequently how brilliant the stone will appear. A poorly cut diamond will actually loose light and appear dull. Diamond cutters do their utmost to fashion each stone according to an exact mathematical formula, in which the facets of the diamond are polished at precise angles to one another. The most famous formula for the 57-facet round brilliant stone was introduced in 1919 by a Belgian mathematician, Marcel Tolkowsky. 

Cut is arguably the most difficult of the Four Cs to judge, and most grading labs, with several notable exceptions, do not go beyond assigning a description of “poor,” “fair,” “good” or “excellent” to cuts various components, such as proportions, symmetry and polish. Proportion would refer to the angles of inclination and proportional relations between the various parts of the stone. Symmetry refers to both linear and angular deviations. Polish refers to external characteristics of the finished diamond, which mostly are the result of the polishing process.