What has a fish got to do with buying a diamond?
When diamonds are cut too shallow, the internal reflections of the girdle would be seen inside the table facet. This creates an image of a grey ring that is distracting and unpleasant to look at.
People in the industry refer to this phenomenon as a fish-eye because it does look very much like one. And trust me, a fish-eye diamond ain’t pretty at all.
It doesn’t matter if the diamond has the highest clarity (IF), best color (D) or other attributes on a grading report that may suggest a “beautiful” looking stone. These kind of stones suffer from a lack of brilliance and look plain ugly. In extreme cases, this cut defect can look as bad as an I3 type of inclusion.
The examples above show diamonds with some degree of fisheye effects in them. The severity of this phenomenon is dependent upon a number of factors such as:
Here’s a quick explanation of why the above factors affect the fish eye. Due to the way light travels, lower pavilion angles would cause a diamond to have less brilliance and make cut defects even more pronounced.
As light enters the diamond, the shallow depth causes light rays to be reflected from the girdle back through the table. This is the reason why a thicker girdles and larger table will make the fish eye more severe.
Yes. Technically speaking, the fisheye effect is caused by shallow pavilion angles and large tables. Shapes like ovals, pears and hearts would also exhibit this effect if they are cut with a bad combination of angles.
This heart shaped diamond shows symptoms of a fish eye forming under the table facet.
To conclude, the fish-eye effect is an indication of very poor cut and will make the diamond dull and lifeless. Unlike subtle problems in clarity or nuances in color, you don’t need to be an expert to notice this issue. If you do see them, avoid them like the plague.