The grading systems most commonly used include GIA (Gemological Institute of America), IDC (International Diamond Council), Scan. D.N. (Scandinavian Nomenclature), and CIBJO (Confederation Internationale de la Bijouterie, Joaillerie, Orfevrerie, des Diamants, Perles et Pierres). All these systems are generally similar. Sometimes, color tints or clarity grades are denoted by different terms, whereas the ranges of these parameters are the same. As a rule, a diamond does not need to be examined repeatedly, and one system can be converted into another using the table. Within the IDC clarity grading system, measurements of the size and brightness of inclusions are accompanied by the correction of the clarity grade by the special table. This makes this system more objective than others. Sometimes, it is difficult to determine if an internal feature is external at the same time. Hence, the IDC system suggests that the clarity grades of the stones with significant external defects should be corrected. Thus, the same stone can be graded variously by grading systems. Nevertheless, the similarity of the systems allows the use of the grades obtained in any of them. The GIA system is increasingly acknowledged all over the world. In particular, the GIA terms used for color grading have almost completely replaced traditional terms.

The GIA system is usually preferred to the Russian system. These systems differ in both basic principles of grading and particular parameters and ranges.

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Within the Russian system (TU), the color grading is performed in a different way for the shapes Single Cut and Full Cut (up to 0.29 ct) and Full Cut (more than 0.30 ct). The description of colors of various groups is of primary importance, while masterstones are rarely used. The terms employed by TU ("color", "tint", "shade", "hardly visible", "slightly noticeable", etc.) can be variously interpreted. Color grading based only on descriptions often brings about unreproducible results. For example, according to the Russian grading system, a small stone with color grade 5 has a deeper color than does a stone with color grade 5 weighing more than 0.30 ct. The Russian system does not have definite terms for color grading of fancy diamonds, and only some rare colors are assigned to the first color grade.

Diamonds of such colors are very rare in Russian deposits, and brown and yellow diamonds, even if deeply colored, are assigned to the last color grades. In the general case, a brown diamond is characterized with a lower color grade than a yellow diamond with the same color saturation.

According to the principles of the GIA color grading system, only color saturation rather than tint is taken into account within the range from D to Z. Therefore, a yellow and a brown stones with the same color saturation are characterized by the same color grade. Additional description may be provided for color characterization; for example, M-brown. The color grade does not depend on the stone size. An independent grading system has been developed for fancy colors. Several tables providing the correspondence between the TU and GIA color ranges were developed in Russia on the basis of the analysis of a great number of stones. These ranges are similar, but do not coincide for all specimens, since, unlike GIA, the TU system distinguishes the brown and yellow series of tints, when coloration becomes obvious. According to the TU system, the highest (first) grade of color can not be assigned to stones with high luminescence, while GIA may characterize such a stone with the highest grade (D) (with subsequent correction of the stone price for the high fluorescence).

Correspondence of color grades determined by TU and GIA systems.

TU	Single Cut	1		2				3										4
	less than 0.29 ct	1		2			3				4					5	6		7
	more than 0.30 ct	1	2	3		4		5		6			7			8		9
GIA		D	E		F		G		H			I		J	K - Z

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The GIA and TU systems are based on somewhat different principles of clarity grading. The GIA system judges how difficult it is for an average expert to detect defects under the loupe or by the unaided eye and if the defects can influence the integrity of the stone. The Russian system involves descriptions of the number, size, nature, and location of defects tolerable for one or another clarity grade and employs the term "transparence" when the pavilion facets are observed through the crown. As in color grading, clarity grading is performed separately for small stones, large stones, and Single Cut. Correspondence of the clarity grades adopted within each of the systems is given in special tables, which, however, do not provide absolute conversion for some diamonds. Such exceptions can be exemplified by a stone with a twinning line as a major feature, which is hardly detectable under the loupe. Such a diamond is graded at 6 by TU and VS1 or VS2 by GIA. The clarity grades 4 and 5 of the TU system do not tolerate such defects as dark pinpoints or inclusions. Hence, if a defect is determined as pinpoint, the stone is assigned to grade 3, but if the defect is classified as inclusion, grade 6 is established. The GIA system can come up with any clarity grade just depending on the size of the defect.

Correspondence of clarity grades determined by the TU and GIA systems.

TU	Single Cut	1	2					3								4						5	6
	less than 0.29 ct	1	2		3			4		5						6				7		8	9
	more than 0.30 ct	1	2		3		4	5		6		7		7a		8		9		10		11	12
GIA		IF		VVS1		VVS2		VS1	VS2		SI1		SI2		SI3		I1		I2		I3

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Russian technological requirements for diamonds have been developed for manufacturing purposes rather than for diamond grading. For this reason, the TU system determines the tolerances for cutting parameters, according to which diamonds can be assigned to grade A or B or fall beyond the TU classification. In the strict sense, diamonds that do not fit the TU parameters cannot be graded within this system. The GIA system allows grading of any diamond by employing the three main criteria: proportions, symmetry, and polish.