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The quality of any diamond piece is determined by its color, clarity, cut, and carat weight, as defined by the Gemological Institute of America, GIA. These are what are together referred to as the 4Cs of diamonds. Flaws, inclusions, cut defects and other imperfections that may affect any of these 4Cs will go a long way in determining the value of diamonds.

Of all the 4Cs, the cut is particular in this regard. The cut of a diamond is concerned with its proportioning, depth, symmetry and polish. This is what greatly affects how much the piece would shine. Thus, the brilliance of any diamond piece is hugely affected by its cut. A poorly cut diamond will not exhibit brilliance.

Amongst other considerations, the cut of a diamond is concerned with its depth. Diamond pieces are to be cut optimally. When a stone is cut too deep or too shallow, defects may occur.


If a diamond piece is cut too deep, it may result in what is known as a ‘Nail head’. A NAILHEAD is a visible black shadow noticed in a deeply cut diamond. When a diamond has a nailhead, light reflects off the bottom at an angle that is obscure, instead of reflection on the eyes directly. Typically, a nail head is noticed as a dark round black shape appearing at the center of the stone. This anomaly is referred to as nailhead mainly because the shape is similar to the head of a nail.

A stone with a nailhead is often poor in outlook and overall cut. Hence, it is expected that these stones will be lower in value, other conditions being constant.

Typically, a stone is supposed to have a pavilion depth of between 40 and 43.2%. When this value increases greatly, the stone is bound to have a nailhead. Nailhead is often noticed in diamond pieces with pavilion depth greater than 48%.


Asides the nailhead, there are so many other common defects that may result in, when a stone is not properly cut. The common property most of these defects have is that they affect the way the stones refract light. In general, these defects have a negative effect on how the outlook of the diamonds turn out. Cut diamonds with defects cannot be identified as perfect cuts.

The common defects in diamond cuts (asides the nailhead) include:

  • Fish eye
  • Bow tie


A fish eye is more like the opposite of the nailhead. While a nailhead is caused by the pavilion of a stone being cut too deep, fish eye is caused by a shallow cut pavilion. A fish eye can also occur in diamonds with overly large tables.

When diamonds with pavilion depths less than 40% are examined face-up, a fish eye is noticed as a circular grey reflection of parts or the totality of the girdle through the table.


Like the nailhead and fish eye, the bow-tie occurs as a result of poor cut. It is different from the nailhead and fisheye effect in that, while these two are caused by leakage of light, the bow-tie effect occurs due to obstruction of light when viewing the stone. The bow-tie occurs in fancy cut diamond pieces like oval, marquise, pear, and heart. It looks similar to a real-life bowtie. The bow-tie appears as a dark area across the body of the diamond. The facets of a diamond stone manifesting this effect will not properly reflect light irrespective of the direction you face it.

Bow-ties can be eliminated or at least, their intensity can be lessened. To do this, diamond cutters make sure to follow laid out angles which are standards. While eliminating the bowtie effect is possible, cutters do not always do so completely for economic and other reasons. For instance, if a cutter notices an inclusion in a rough stone, instead of cutting a stone to standard, following the cut they wish to transform a stone to, may try to remove the inclusion whilst making sure the carat weight is not totally affected. This caution may leave the stone exhibiting bow-tie effect.

Bow-tie effects are not completely bad, especially in fancy cut diamonds. Infact, oval- and marquise-cut diamonds without a slight bow-tie may look dull. Although this is not the same with bow-ties that affect a huge area of the stone.


A perfect cut diamond is also known as an ideal cut diamond. It is a diamond cut that exhibits a good light performance. This is mainly due to the fact that it was cut to the right depth, proportion, symmetry and polish.

Ideal depth 

When depth is mentioned with respect to diamonds what should come to mind is the depth of the pavilion as well as the height of the crown. An ideal cut diamond should have the right crown height and pavilion depth. The depth of a diamond is measured in percentage and there are certain existing standards for perfectly cut depths.

Ideal proportion

Diamond proportion refers to the ratio of each part of a diamond with respect to the overall diameter of the diamond. The proportion is concerned with the specific lengths into which each facet was cut, the size of the table and other parts of the diamond.


Diamond stones are cut in relation to depth, proportion, symmetry and polish. Over the years, diamond cutters have set standards which ensures that the perfect cut diamonds are obtained. When diamonds are not cut to the right shape with respect to depth (e.g of pavilion, crown), proportion, and symmetry, defects are bound to occur. 

A nail head occurs when the pavilion of a stone is not cut to the right depth. It happens when the pavilion is cut too deep. A fish eye on the other hand, occurs in the opposite case. That is, when the pavilion of a cut diamond is too shallow. Other defects too can be noticed when standards are not followed or when errors are made during cutting.

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