Fisheye

Fisheye
Diamond with a pavilion depth of less than 40 percent, in which a circular gray reflection of all or part of the girdle appears through the table when the stone is examined face-up.
Fluorescence ...

Fisheye: A diamond whose pavilion (bottom) is exceedingly shallow, producing a glassy appearance and a noticeable lack of brilliancy. Also called shallow, spread or swindle cut.

Fisheye
A diamond with exceedingly shallow pavilion, which produces a glassy appearance and a noticeable dearth of brilliancy.
Fissure ...

Fish Eye, Fisheye
A detrimental optical effect in diamonds which are cut too shallow, particularly with a shallow pavilion, or with an overlarge table.

aka Fisheye Jasper, Orbicular Jasper
This gemstone is silicified (transformed into silica) rhyolite flow from the northwest coast of Madagascar.

Stones that are less than 57% can be "fisheyed", while those greater than 64% will tend to show a dark spot (round cuts) or a darker "bowtie" (marquises, pears, hearts, ovals) centered on the culet.

Note to § 23.15: Stones that are commonly called "fisheye" or "old mine" should not be described as "properly cut," "modern cut," etc.
§ 23.16 Misuse of the words "brilliant" and "full cut." ...

As shown in the diagram below, if a round brilliant diamond is cut too deep (nailhead) or too shallow (fisheye) light leaks out of the pavilion and does not return to the observer's eye. In diamonds with too steep a crown angle ( ...

With a shallow pavilion, it will display a light hole or a fisheye effect at the center due to the absence of internal reflection.

A diamond that has been cut too deep will often have a dark center called a nailhead. Diamonds that have been cut too shallow will often display a white circle around the table edge which is commonly referred to as a fisheye.

See also: Cut, Clarity, Stone, Table, Diamond