When assessing a diamond’s quality, the 4 C’s provide an excellent foundation and basic overview. Yet the factors that contribute to a diamond’s value go well beyond them. A shrewd gemologist or experienced diamond expert looks beyond the 4 C’s at subtle details that make big differences in the overall quality and worth.

In this section, we will outline a few key attributes that play a big role in choosing the right diamond. So, read on to learn some very important secrets of the gem trade that will help you in ensuring that the diamond you choose will be as beautiful as possible and maintain or grow its value.

Depth Percentage

A diamond’s depth percentage is found by taking its height (in millimeters), measured from the culet to the table, and dividing it by the width or diameter of the diamond. For example, a diamond that is 4mm in depth and 6.5mm in width has a depth percentage of 61.5%. On a grading report, there are normally two measurements of depth – the first is the actual depth measurement in millimeters (shown under ‘measurements’ at the top of a grading report), and the second is the depth percentage, shown in the diamond graphic, which shows how deep the diamond is in relation to its width.

The depth percentage is important because it will affect the way your diamond processes the light that comes in contact with it. As light meets a diamond it either reflects (bounces back) or refracts (bends while passing through the facet). The angle at which the light hits the facets determines whether the majority of light reflects or refracts, which is why cut is so important. If the diamond cut is too shallow, entering light passes through the facets (refracts) and escapes through the bottom of the diamond. If the diamond cut is too deep, entering light will reflect at an angle that is too low, causing the light to refract (pass through the facet), and escape through the bottom of the diamond. In a well cut diamond, the light reflects off of the facets and back to the crown at a low angle, creating a scintillating and fiery refraction as it exits through the top and to the observer’s eye.

The ideal depth percentage varies with the shape of the diamond. A depth percentage that may be too much for one shape might be essential for another. For instance, a princess cut with a 75 or 77 percent depth percentage would  be considered acceptable and can yield an attractive diamond. On the other hand, a depth of 65 percent for a round brilliant cut diamond would be excessive and would likely be detrimental to its beauty. We’ll provide specific recommendations for each shape of diamond later in the Diamond Shapes section.

Table Percentage

The table refers to a diamond’s largest facet, the flat surface at the very top of the diamond, which can be seen when the stone is face up. The table percentage of a diamond is calculated by dividing the width of the table facet by the overall width or diameter of the diamond. For example, if the table facet is 4mm wide, and the diamond is 6 mm wide, its table percentage is 66.7%.

The table plays a vital role on brilliance and light performance of a stone. It’s main purpose is to refract light rays entering the diamond and allow reflected light rays from the pavilion facets to pass back into the observer’s eye. If the table is too large, then the crown will be too small, restricting the amount of dispersion and fire a diamond showcases. On the other hand, if the table is too small, a larger crown will disperse light incorrectly, resulting in reduced brilliance. Therefore, it is very important to choose a diamond with a table percentage that creates a balance between light transmission through the table and color dispersion through the crown and upper girdle facets. In the Diamond Shapes section, we will provide specific recommendations regarding table percentage and other characteristics for each shape.

A note regarding depth and table percentages

Together, the depth and table percentages of a diamond can be a good indication for brilliance and value (stones that are cut too deeply are usually cut to retain weight from the rough). It shows you how the stone has been cut in relation to its proportion and is usually one of the first few values to help you instantly weed out diamonds with less ideal cuts. But they are only one chapter in the story of a diamond - one ‘facet’, if you will, in a wide array of contributing attributes. And because the overall Cut grade already incorporates both factors, it should be used as the primary determinant when choosing a diamond.

Additionally, a diamond should never be chosen strictly based on its ‘data.’ There are limitations imposed by numbers on a paper and, on countless occasions, two diamonds with identical specifications will seem to have entirely different attributes and personalities in person. Nothing replaces the seasoned eye of an educated diamond and gemstone expert.

Fluorescence

Fluorescence is the visible light some diamonds emit when they are exposed to invisible ultraviolet (UV) rays (95% of the diamonds that exhibit fluorescence, the color seen is blue. In rare instances, the reaction is yellow, white or another color). On a GIA diamond grading report, fluorescence refers to the strength or intensity of the diamond’s reaction to long-wave UV, which is an essential component of daylight.

Fluorescence is quite common, with approximately 25-35% of diamonds exhibiting some degree of it. But, with only 10% of those (or 2.5-3.5% of all diamonds) showing strengths of fluorescence that may impact appearance (i.e., strengths noted on laboratory reports as medium, strong or very strong), it has no widely noticeable effect. Only in rare cases do some diamonds with extremely strong fluorescence appear hazy or oily; fewer than 0.2% of the fluorescent diamonds submitted to GIA exhibit this effect. And it does not compromise the structural integrity of a diamond in any way.  

So why are we talking about it here? Because you may be able take advantage of this phenomenon by choosing to purchase a diamond that includes some fluorescence. Despite its effects only being a factor when viewed under UV light (planning on going to any raves?), diamonds that exhibit some of this attribute can be up to 10-20% cheaper than those that do not, due to market forces and demand - making this a great hack for stretching your budget. Just make sure to steer clear of those with ‘strong’ or ‘very strong’ gradings and always have it reviewed in person by either yourself or a diamond expert.