Formerly, the scintillation in gemstones has been defined broadly as alternating display of refiections from the polished facets of a gemstone; it is a flashing or twinkling of light from the facet. However, for deeper consideration of the phenomenon, we should take into consideration that there are some higher radiations from facets. We define that the scintillation consists of radiation and reflection from facets, the former being the leading part. In evaluating brilliancy of diarnond, the inclinations θ (angle between optical axis and radiant ray) of a gemstone are derived from three cases (table→table, main→table, table→main; Fig.22). The changes of radiation area by the inclination of a gemstone which is based on the F-Cut I (cf. Table 2) are illustrated in Fig.23, and the changes of F- and S-values presented graphically in Fig.24. The best F-value (F'_1, F"_3) or convergent ray becomes the flash and then finally produces dispersion or fire. The twinkling can be defined as a momentary alternating display of the ray from some facets, i. e. high and low of the illumination, emanation and disappearance of the radiation, expansion and contraction of the radiation area. Of monocular and binocular vision, the latter gives higher scintillation. It can readily be conjectured that a combination of a good many facets and twinkling by a numerical image misdirects the source of the scintillation. For reference, Tolkowsky-cut and Quartz are presented graphically in Fig.25,26 and F-value. We have introduced some expressions of relation for facetting as an Appendix with Fig.28.
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