Fish Eye Effect in Diamonds

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A diamond’s optical performance is defined by a whole range of different factors, including its table and depth percentages, cut, polish and symmetry grades and pavilion angle. These together will define the overall look of the diamond, so finding their balance is vital.

While it has a lot to do with sciences of nature as well, diamond cutting could better be defined as a branch of arts. For that reason, in spite of all the jeweler’s carefulness, the final gem stone will sometimes become tainted by an optical phenomenon: the stone will appear dull and glassy beneath the table, like a dead fish’s eye.

What is the fish eye effect in diamonds?

Contrary to what the name might suggest upon first hearing, the fish eye effect in diamonds has remotely got anything to do with the types of optical lenses with the same name. In fact, the only similarity between the two is that both have something to do with lights. However, while the latter collects light and creates a unique image of things, the first one is caused by light, more exactly it’s reflections within the diamond.

The technical definition for this rather double-edged phenomenon is the following: a reflection of light just inside the table of the girdle on the opposing side. It appears as a circular inclusion, which in extreme cases can look like as bad as an I3 grade inclusion.

You can see from that why buying an affected diamond could be one of the worst possible ideas (with a few exceptions being present in this case too): these kinds of stones suffer a huge quality drop, even though their clarity and other important characteristics would suggest otherwise.

When does the fish eye effect occur?

Fish eye effects occur only in shallow cut round diamonds, with the stones’ pavilion angle situated somewhere between 39 and 41 degrees and proportionally the table percentage is  between 58,4% and 72,2%. In layman terms, the diamond is too shallow and the table is too large.

The girdle’s thickness and polish grade can also seriously vary the severity of the optical errors in the diamond. Sometimes the diamond’s culet can also take part in the formation of the fish eye effect, however this happens very rarely, and when it does, it is caused by an overly large culet. In milder cases the issues are only visible when watching the stone exactly from above, while gradually severing cases become visible from a very slight angle to all angles.

Diamonds that present the fish eye effect are rated negatively in terms of their clarity, treating the effect like an inclusion, because that is how it appears to the naked eye. To avoid diamonds with fish eye effects, you can easily weed them out based on the diamond’s proportions and stick within a certain ideal range.

Should I consider buying a diamond affected by the fish eye effect?

Although it would seem as a stupid question, the answer is, as always, a little bit complicated. The general rule is that you should never buy a diamond with a fish eye effect so severe that it is visible from every angle. However, what you should consider is buying a stone with a very mild severity occurrence, preferably where the effect presents itself when looked at only from the top or just from a very slight angle.

Affected diamonds have very cheap carat price compared to otherwise similar, unaffected stones, so if you are going for the size of the stone rather than its optical performance, then a slight affection might be just the thing you are looking for.

If you are one of those people who feel more impressed by size than the stone’s brilliance, a deal of this kind would work perfectly for you, but in the case of such purchases you should always insist to be able to inspect the diamond personally.