This paper describes the principals and development of the HCA system for predicting the beauty and desirability of round brilliant cut diamonds1 based on facet proportions.
We begin with an observation that I made several years ago concerning the effects of an inverse relationship between crown and pavilion angles, followed by an examination of some relevant previous studies and a brief discussion of some tools used in the methods section; the Fire Scope, GilbersonScope and DiamCalc.
Brilliance, light return, fire, scintillation are defined and a discussion of the methods used to create the values for the three beauty components of the HCA system are explained.
The fourth HCA factor: spread relates to a diamonds size, a major factor in considerations of desirability.
Applications of HCA are discussed next and a zone of poorly performing crown and pavilion proportion combinations, or no go zone (NGZ), not previously mentioned in the literature, is identified. Most of todays diamonds are cut to proportions in this zone. The result is light leakage out the pavilion that can be seen through the table in a face up view. This zone includes many stones in the top cut grades of most labs cut standards. A discussion of the shortcomings of existing cut grading standards follows.
Suggestions for future diamond cut grading standards are made based on matching the various demands of different people, including those with different eyesight and expectations.
Finally the limitations of the accuracy of measuring instruments and proportion information (especially the pavilion angle) are discussed.
1 This paper concerns round brilliant cuts. Some principals may be relevant to fancy shaped diamonds.