When a diamond is unearthed, it exists as a misshapen stone waiting to be cut and polished to perfection. Gemologists study the internal lattice of the stone, analyzing its ability to interact with light so that they can identify the best cut. The idea is to bring out the best visuals, the perfect shine.

However, if the diamond is cut too shallow or too deep, a phenomenon known as the “fish-eye” or “nail-head” comes into effect. It is most noticeable in round cut diamonds but may also appear in other diamond cuts if the combination of angles isn’t balanced.

Before we move on to what the fish-eye or the nail-head looks like and why is it formed, you might want to refresh some basic terminologies used for diamonds.

The Ideal Diamond

Ideal Cut Diamond

When you look at a diamond face-up, what you see is an interesting combination of reflected and refracted light. An internally flawless, ideal cut diamond would do so perfectly, creating a magical effect. Inclusions appear as dark (or white) spots that fail to reflect light.

When light enters a diamond, it is supposed to reflect internally in a way that makes the stone appear bright on the inside or, simply put, sparkle! The pavilion angles and the table are set proportionately to facilitate this reflection. This is called the ideal cut, which was suggested by a great Polish mathematician and diamond cutter – Marcel Tolkowsky.

Cut Too Shallow or Too Deep

Fish Eye Diamond

However, if the diamond is cut too shallow, that is, the pavilion angles are smaller than the ideal cut, the diamond will begin to leak light at the bottom due to its inability to reflect internally.

Conversely, when the diamond is cut too deep, that is, the pavilion angles are larger than the ideal cut, light rays will be reflected internally and eventually lost through the other side.

Such diamonds are said to be leaking light, appearing duller and less sparkly than they should.

But that’s not all. The girdle of these diamonds also reflects on the core, forming an impression on its pavilion when observed face-up. Shallow cut diamonds will have a dull ring-like structure visible in the center of the stone, called the fish-eye for its resemblance to the fish’s eye.

Nail Head Diamond
Nail Head Diamond

For deep cut diamonds, the core would appear to have a centrally located dark spot, its size proportionate with the pavilion to table ratio. This flaw is known as the nail-head.

To Sum It All

In short, the angles on the pavilion are unable to reflect light like they’re supposed to do and hence diminish the beauty of the stone. It is the responsibility of the diamond cutter to ensure the cut angles are closest to the ideal cut for optimal visual performance.

Generally, a diamond with fish-eye or nail-head flaws are degraded by the gem community and are therefore labeled as dead stones. Severe fish-eye or nail-head may lead to a “Severely Included” rating, making the diamond unusable. So beware of anyone who tries to sell you a fish-eyed or nail-headed stone. They’re an absolute must-avoid-at-all-costs.

So what would you be willing to tradeoff – sparkle or money? Let us know in the comments section below.

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