A fish-eye is a girdle reflection seen just inside the table. When light travels through the girdle and reflects from a flat angled pavilion, the image of the girdle is transmitted to the table edge.
The Three Components of a Fish-eye:
- Flat pavilion angles (shallow pavilion depth)
- Girdle reflection inside table
- Large culet (sometimes)
The flat pavilion angles cause (1) the dead center (little brilliance and dispersion), and (2) an image of the girdle inside the table. The table size has a direct effect on how easily the girdle reflection can be detected, or to what degree the stone must betilted before seeing the reflection. (3) A large culet can be the last feature of a fish-eye, but it is not essential.
A large culet affects the pavilion depth, but not the pavilion angle. The larger the culet is, the shallower the pavilion depth becomes. However the pavilion is really what we are concerned with and the depth percentage is merely a tool by which to correlate the two numbers. (A 40.75 ° pavilion angle is approximately a 43% pavilion depth). A large culet's negative influence on the appearance of the stone is an eye-visible hole in the center of the table. This becomes even more evident when adding the lack of brilliance and dispersion that flat pavilion angles allow.
To summarize a fish-eye's anatomy: Consider the dead center as the major part of the eye's surface, the girdle reflection as the outline of the eye, and the large culet (if present) as the pupil.