1. Abstract
2. Introduction
3. Grading the symmetry of round brilliant cut diamonds in Russia
4. Grading the symmetry of round brilliant cut diamonds at HRD (Belgium)
5. Grading the symmetry of round brilliant cut diamonds at GIA (USA)
6. Composite table of symmetry distortion grading
7. Variety of problems that arise when using the current symmetry grading systems
8. Symmetry axes of a diamond
9. Axial symmetry matrix
10. Major and minor symmetry distortions
11. Conclusion
12. References

Abstract

The approach to grading the symmetry of round brilliant cut diamonds, developed in Russia, Europe, and USA, is considered. Knowledge on diamond cut technology is used to analyze the reasons of some important cases of asymmetry. A more objective method is proposed for grading these cases. The method implies composing a matrix of mutual axial deviations on the basis of a scanned 3D model of the diamond.

Introduction

When grading a diamond, one of the key characteristics is the symmetry of the stone. Though there is no conventional definition what the symmetry of a diamond is, the approach adopted in different countries and used in various grading systems implies fixing and grading certain deviations of its geometry, pattern, and visible image from the ideally regular, symmetric ones. These deviations may be either observable with the naked eye or with a 10x loupe, or detected by some measuring devices. A wide spread of various measuring devices, including those using a diamond scan technology, raised the question "How the deviations measured by such a device should be interpreted in the framework of a certain grading system developed earlier?" Another question arose when structural illumination schemes, such as "Firescope", "Idealscope", or "Hearts and Arrows Viewer", became popular. The pattern observed under such illumination conditions allows the observer to estimate the visual symmetry or asymmetry of the diamond under study. However, the data obtained in such a way do not correlate with symmetry grading tests performed in gemological laboratories. This caused the concept of "optical" symmetry to appear, which is not the same as traditional symmetry. Consider the existing approaches used to grade the symmetry of a diamond. Starting from this point, let us confine ourselves to a single diamond shape, namely, the round brilliant cut with 57 facets. This cut is most widespread and, ideally, it has an 8th-degree symmetry axis. This is the cut, whose symmetry the most strict requirements are applied to, while grading the cut quality and symmetry of this particular shape is best developed by gemologists. When grading the symmetry, different distortions are usually divided into more and less important ones. For example, the Gemological Institute of America distinguishes between "major" and "minor" symmetry. This paper is mainly focused on considerable distortions of symmetry and the necessity to improve grading these.