The main factors that determine the value of the cut diamond are: weight , color , purity or better the absence of imperfections and cutting proportions . Between 1940 and 1950, GIA (Gemological Institute of America) developed the International Diamond Grading System and the 4 C as a measurement system to compare the qualities of different diamonds on the market.

1. Carat (weight)

The carat is the weight measurement unit of diamonds and other precious stones, it corresponds to 0.20 grams. In the case of gems weighing less than one carat, the weight is expressed in units 100 times smaller, ie points (for example: 50 points = 0.5 carats). Obviously, the more a diamond is of considerable size, the more it reaches a high carat and consequently a high value.

2. Clarity

The degree of purity is determined by quantization of the internal and external characteristics which, by their presence, disturb the brilliance. Considering the set of internal and external features, their size, position, and number, a ten-degree purity scale ranging from FL (flawless) to I3 (included 3) is defined.

3. Color

There are colorless, blue, green, yellow, brown, pink, purple, orange and black diamonds. Generally they have a yellow or brown color of varying intensity while the colorless stones are rare. The value decreases as the yellow or brown tones grow when the tone becomes so deep that it is of value.

The value of the gem decreases as the shades of yellow, brown and gray increase. The classification of color shades (from D to Z) is done by colorimeters or by means of sample stones or “master stones”.

Shades of diamond colors.

The stone to be examined must always be placed on both sides of the comparison stone because the human eye perceives two objects of the same identical shade, the one on the left as slightly darker.

4. Cut

The choice of cutting is generally conditioned by the need to achieve maximum brilliance and dispersion with the least weight. To have an optimal response to light (refraction, reflection, brilliance, glitter and fire), a diamond must be cut perfectly, complying with rigid mathematical calculations that generate geometric designs, dimensions and angles required for facets. The measure of proportions takes on a great importance in the certification, as it is a feature that substantially affects the value of the stone.

The most commonly used traditional cut is the round brilliant, consisting of 57 or 58 facets. There are other types of cut used for diamonds: drop, heart (59 facets), shuttles or marquises (58 facets), emerald (48 or 50 facets), oval (56 facets), princess (76 facets) radiant (62 or 70 faces).

The brilliant cut is certainly the most important for diamond, in fact commonly used as brilliant as a synonym for diamond. The shape of the brilliant cut is generally made up of two pyramid trunks assembled for the largest bases, of the same size. The upper pyramid is called the crown and, starting from the common base of the two pyramids, occupies a height corresponding to a third of the total height; the lower pyramid in the pavilion occupies the remaining two thirds of the height of the two pyramids themselves. The crown is delimited: at the top, by an octagonal face said upper table; down, from the common base whose contour is called the belt. The pavilion starts in turn from the common base of the two pyramids and ends at the bottom with an octagonal face truncating the pyramid, said lower table (or culasse). The proportions that allow for maximum brilliance and dispersion at the same time have been calculated by considering the appropriate relationship between the table size and the angles that the crown and pavilion form with the belt plane. The proportional measurement allows a quick measure of the magnitudes that define the proportions of the bright.

To determine the diamond value in relation to the quality of the cut, we refer to three factors: proportions, symmetry and polish.

Thus, four categories of symmetry were created: Very Poor , Poor or Fair, Good and Excellent .

You can add a fifth , often overlooked and source of misunderstanding, namely  Fluorescence, to these parameters .  The fluorescence index is inversely proportional to the diamond value, so the diamond will be fluorescent, less it will be valuable.