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HCA - Defining the Beauty and Desirability of Round Brilliant Diamonds

Fish-eye Adjustments and Comments

A ‘fisheye’ appears as a circular inclusion just inside the table, and in extreme examples can look like an I3 inclusion. The fisheye effect is a reflection of the girdle on the opposite side. Diamonds that display a fisheye effect are given a penalty score and a warning comment that a diamond is, or almost is, a fisheye.
 
Fisheyes are more apparent if the pavilion is shallow and the table is large, or a combination of the two. If the girdle is very thick and not polished (bruted) a fisheye looks far worse. HCA has no input for this information.
 
Fisheyes require no tilt to be seen if a diamonds pavilion is 39° and the table is 58.4%, an increase in table size to 59.4% in this example would result in a fish-eye that would be of a width of 1% of the diamonds diameter. Diamonds with deeper pavilions will still show a fisheye if the table size is large enough. For example a 41° pavilion and 72.2% table shows a fisheye. Some people are happy to accept a near fisheye because the stone will have an excellent spread. An automated comment on the cut adviser system warns a buyer if a diamond is close to showing the fisheye effect.
 
We may change the method of calculation to an angle based calculation. The results would be very similar.

Table 8. Fisheye penalties

	Fish - eye		Near Fish - eye
Percentage	0%	1%	2%	3%	4%	5%
Penalty score	1	0.8	0.6	0.4	0.2	0
Comment	This diamond shows a fish-eye effect. Do not buy it.		A fish-eye effect is visible just under the table when viewed from a slight angle.		A fish-eye effect may be visible just under the table when viewed from an angle.

Figure 19. Fish-eye ray path
           
 

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