Determination of the cut quality of a diamond and visual evaluation of its beauty

The factors that determine the cut quality of a diamond and the visual evaluation of its appearance can be divided into three main groups:

1. Parameters of light sources that characterize the illumination of the diamond:  

• Source type (its radiation spectrum, luminance, and angular size)
• Source position
• The number of such sources.

2. Optical effects exhibited by the diamond itself and its optical properties:  

• Light return - the capability of the diamond to return a fraction of the incident light to the observer's eye
• Fire - the capability of the diamond to disperse a white light into iridescent colors perceived by the observer
• Color - the result of the dependence of the light absorption factor of the gem upon the wavelength of the incident light
• The pattern of facets and highlights within the diamond and their symmetry
• "Dynamic Contrast" - the rate of change of the diamond appearance while it is rotated
• Clarity - the preferable absence of internal inhomogeneities that absorb or scatter light
• The flatness of the facets, the quality of their polishing, and the surface smoothness

3. Viewing conditions:  

• The observer's position with respect to the diamond
• The psychophysiological features of the observer:

• • vision stereoscopy
  • pupil motion
  • color perception, effect of background
  • eye's resolution
  • eye's sensitivity
  • spatial and temporal (the difference in successive views of the gem) contrast
• The psychological criteria for evaluating the diamond beauty. The existing stereotypes and the fashion of the market.

The correlations between some of the above factors are given below:  

• Diamonds cut similarly but having not equal size are perceived quite differently. Smaller diamonds are usually cut so that to have fewer facets. The influence of the size of a gem upon its fire is most strong.
• The radiation spectrum of the illumination source determines the perceived color of a diamond and affects its fire as well. The color depends on the product of the radiation spectrum of the source by the transmission spectrum of the gem. Both multipliers are equally important. The coloration of the diamond, as a rule, weakens its fire. In some cases, intensive coloration may completely eliminate the fire.
• The angular size of the illumination source strongly affects the fire of the diamond;
• The positions of multiple illumination sources and their total number determine the " Dynamic contrast" and the pattern of highlights.
• Poor polishing reduces both fire and light return of a diamond;
• So do internal defects of the gem;
• A snapshot of a diamond is not adequate to its image visually perceived by an observer;
• The cut of a diamond should be evaluated under those illumination conditions being peculiar to the place where it is used ");
• Traders are prejudiced against non-typical cut parameters and facet patterns;
• The degree of symmetry of a diamond strongly affects its beauty.
• It is inadmissible to improve the appearance of a diamond (as evaluated by an observer) by considerably decreasing its weight;
• For the evaluation of the cut quality of a colorless or weakly colored gem, its light return is not so important as the fire and " Dynamic contrast ".