The main parameters of diamond grading are represented by four :
  • Carat weight
  • Colour
  • Clarity
  • Cut

Each of these four parameters affects the price of a diamond. Therefore, appraisal is impossible without careful grading. Working with diamonds, an expert successively carries out the following procedures.

Performing diamond grading, an expert fills in the worksheet (testing records) which usually contains the following data:

  • Full name of expert, date, and stone number;
  • Results of stone identification;
  • Shape and cut;
  • Weight in carats, measured or calculated;
  • For round diamonds: min and max girdle diameters and average diameter (mean value in millimeters);
  • For fancy shapes: length and width (mm);
  • Total depth of a stone (mm and %)
  • Proportions: table size (mm and %), crown angle (degrees), crown height (%), girdle thickness (qualitatively and in mm), pavilion depth (%), pavilion angle (degrees); and culet size (qualitatively);
  • For fancy shapes: length to width ratio and shape appeal;
  • Symmetry irregularities;
  • Clarity characteristics;
  • Luminescence: color and intensity;
  • Diagram with internal and external characteristics, symbol key;
  • Comments

When the proportions of a stone are calculated, the average girdle diameter (or width for some fancy shapes) is taken as 100%. Some parameters of stone proportions are interrelated, for example:

  • crown angle, crown height, and table size;
  • pavilion angle and pavilion depth;
  • crown height, girdle thickness, pavilion depth, and total depth (%).

For this reason, some grading systems eliminate the measurements of some parameters if these parameters can be derived from other data.

When the girdle thickness is estimated in percentage, it should be noted whether the narrowest or the widest parts of the girdle are measured.

Luminescence grading often involves only determination of luminescence intensity, while its color is not graded.

To establish the effect of all cut irregularities on the diamond price, some cut parameters are described with tolerance values. For instance, within the GIA grading system, symmetry and polish are graded as "excellent", "very good", "good", "fair", and "poor". Such grades are usually applied to fancy shapes, because the effect of the geometry and pattern of a stone on its fire and the perception of the specimen as a whole is little studied, and no common standards are used for fancy cut styles.