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Diamond Anatomy: Must-Have Knowledge for Purchasing Diamonds

Understanding diamond anatomy...

It’s easy to see why we love the fire of diamonds so much, but the science behind it is much more complicated. Difficult to grasp, diamond anatomy and the 4Cs often confuse first time buyers, leading to bad purchases and regret.

While the most important factor in buying a diamond is that YOU love it, it doesn’t hurt to understand what you should be looking for when shopping online or in a store. Diamonds have specific characteristics that are used to grade its sparkle, such as a diamond’s individual parts and how they interact with one another to create its bright shine.🌟

What is diamond anatomy & why should you study it?

The “anatomy” of a diamond is the collection of all its physical parts and dimensions and how they relate to one another.

How each part and its proportions connect to one another impact a diamond’s cut grade. 👉🏻This is because the measurements of individual pieces of its anatomy affect how brilliantly it will sparkle. Each cut or facet influences how light interacts with the stone, affecting its fire and clarity.

Study Diamond Anatomy: Parts, Proportions, Symmetry and More

The measurements for a diamond’s parts and ratios vary based on the gemstone’s cut and shape. For instance, a brilliant-cut round stone has different expectations than an emerald cut diamond, and the cut quality of both depends on their dimensions and how much each facet fits the industry standard. Let’s discuss how a diamond’s anatomy affects its brilliance and what to look for when purchasing a diamond.

In this article, we will cover:

  • The parts of a diamond
  • Diamond proportions
  • Diamond symmetry
  • Diamond fluorescence

What are the parts of a diamond?

Diamond anatomy consists of multiple, complex facets and parts that affect the 4Cs of a gem. Here are the common names for parts that you may see on grading reports and hear gemologists speak of:

Crown

Diamond Anatomy: CrownA diamond’s crown rests on top of the stone and includes everything above the girdle: the table and the star, bezel, and upper girdle facets.

Table

Diamond TableThe large, flat surface facet on the top or face of a diamond is known as the table. It is the largest facet on a cut diamond, and it is the surface through which wearers view a diamond’s fire.

Star Facets

The Star Facets of a DiamondThe star facets surround the table, sitting just below it. The number of each type of facet may vary depending on the cut and shape of the diamond.

Bezel Facets

The Bezel Facets of a DiamondBezel facets are kite-shaped, and they are nestled between the table and girdle as well as the star and upper girdle facets.

Upper Girdle Facets

The Upper Girdle Facets of a DiamondSitting right above the girdle, these triangular facets are directly below the star facets.

Girdle

Diamond GirdleThe widest part of a diamond, the girdle is the outermost edge of the stone. This is where setting prongs sit, as it is between the crown and the pavilion and provides a secure hold without diminishing the stone’s brilliance. 📝The girdle may also be faceted, polished or “bruted” which means it is unpolished.

Pavilion

The Pavilion of a DiamondConstituting a majority of a diamond’s weight, the pavilion includes all of the stone’s parts below the girdle: the lower girdle and pavilion facets as well as the culet.

Lower Girdle Facets

Diamond Anatomy: Lower Girdle FacetsResting just below a diamond’s girdle, the lower girdle facets are the highest surfaces of the pavilion.

Pavilion Facets

The Pavilion Facets of a DiamondThe pavilion facets are the lowest surfaces of the entire pavilion. They sit adjacent to the culet.

Culet

Diamond CuletThe culet is the lowest area of a diamond. It can either be pointed or faceted: gem cutters often facet a culet to protect the pavilion from damage. However, once a diamond is set, a faceted culet becomes less important as the setting itself offers protection.

Diamond proportions: what are they and how do they affect value?

Each part of diamond anatomy is painstakingly cut by professionals, and their precision affects its cut quality. Every part must meet certain measurements or requirements to grade well, and there are equations and ratios used to determine the craftsmanship used in a diamond’s cut.

✒️There are 10 diamond proportions that affect cut and impact a gem’s quality rating. They’re explained below:

Different Proportions of a Diamond (GIA)
Diagram: GIA (Gemological Institute of America)

Total Depth

This measures the overall depth or height of a diamond, from the top of the table to the bottom of the culet. It’s typically illustrated as a percentage of the average girdle diameter. The equation is as follows:

Total Depth % = Total Depth(mm) / Average Girdle Diameter x 100

The ideal total depth percentage varies by shape and cut:

Shape Percentage (%)
Round 59 - 62.6
Oval < 63
Heart 56 - 66
Marquise 58 - 62
Princess 68 - 74
Emerald 61 - 68
Cushion 61 - 68
Radiant < 67
Asscher 61 - 68
Pear < 68

Table Size

Table size measures the length and width of a diamond’s largest facet: its top, horizontal surface. The width of the table is then used to calculate the table size percentage or average table size, expressed as follows:

Table Size % = Table Average Diameter / Girdle Average Diameter x 100

💡Ideal table size varies by shape, but it has a crucial effect on a diamond’s sparkle. If a table’s width is too wide, the diamond will appear flat and dull, as light will not reflect off the facets properly. In contrast, a short table will make a diamond look too round and trap light inside the stone.
Shape Percentage (%)
Round 54 - 57
Oval 53 - 63
Heart 56 - 62
Marquise 53 - 63
Princess 69 - 75
Emerald 61 - 69
Cushion < 68
Radiant 61 - 69
Asscher 61 - 69
Pear 53 - 65

Crown Height

The crown is the upper section of a diamond, measuring from the top of the girdle to the top surface of the table. Average crown height can influence how well light disperses through a diamond, as the upper facets act as “windows” for light.

Crown height is listed as a percentage, related to the average girdle diameter:

Crown Height % = Crown Height (mm) / Average Girdle Diameter x 100

Crown Angle

The bezel facets sit along the girdle, and the measure between the top of these facets and the girdle is known as the crown angle. It affects the face-up appearance of a diamond and provides a path for exiting light as well as additional refraction.

A well-cut crown angle usually measures between 31.5 and 36.5 degrees.  It works in tandem with the pavilion angle, meaning if the pavilion angle is too shallow or steep the crown angle can be adjusted to compensate.

Star Length

Star length measures the length of the star facets from their tip to the edge of the table. Relative to the distance between table and girdle edges, a diamond with a quality cut will measure between 40% and 70%.

Girdle Thickness

Girdle thickness is a measurement of the band at the center of a diamond, and it is more likely to affect value via weight rather than sparkle. If the girdle is too thick, it will cause a disproportionately heavy carat weight and make a diamond appear smaller. However, if it’s too thin, it is susceptible to damage and chipping.

The GIA grades girdle thickness on a scale ranging from “Extremely Thin” to “Extremely Thick”. Ideally, a diamond’s girdle thickness is somewhere from “Thin” to “Slightly Thick”.

Lower Girdle/Half Facet Length

Measuring the length of the lower girdle facets relative to the pavilion, it creates a diamond’s contrast and dictates brilliance. In fact, if a diamond has slightly longer lower half facets, its scintillation will be stronger. A quality cut diamond typically will have proportions of 65% to 90%.

Pavilion Depth

Calculated as a percentage of the average girdle diameter, pavilion depth is the height between the bottom of a diamond’s girdle and its culet. It can be written as follows:

Pavilion Depth % = Pavilion Depth (mm) / Average Girdle Diameter x 100

If the pavilion is too shallow or deep, light will not shine through the diamond and will instead escape from the sides or bottom of the stone.

Pavilion Angle

The pavilion angle is the angle between the bottom of the girdle and the edge of the pavilion. It greatly affects a stone’s sparkle, as a diamond with a pavilion angle that is too large or too small can appear dark or glassy respectively. If all other proportions are in alignment, a diamond should receive an “Excellent” rating is the pavilion angle is between 40.6 and 41.8

Culet

Pointed or faceted, the tip of a diamond does have some influence on the gem’s face-up appearance. It is the average width of the facet, expressed as such:

Culet Size % = Culet Size (mm) / Average Girdle Diameter x 100

Culets are graded on a scale from “None” to “Extremely Large”, and “Excellent” culets tend to grade as “None” to “Small”.

Diamond symmetry: defects and how to spot them in your report

Another crucial factor in a diamond’s cut is its symmetry. Symmetry is the alignment of a diamond’s facets, and precise size and positioning are important in higher grade diamonds.

Light bounces off the angles and surfaces of a diamond, and proper diamond symmetry ensures wearers can see the displays of colors and flashes through the diamond’s table.

Since not all defects are visible to the naked eye, gemologists use 10x magnification to assess diamond symmetry. After gemologists account for all of their deviations, diamonds receive a grade ranging from “Poor” to “Excellent”.

Here are a few factors that can affect a diamond’s symmetry with some of their abbreviations within a diamond symmetry report:

Off-Center Table

The Defect of Diamond Symmetry: Off-Center Table
Diagram: GIA

The table of a diamond must sit centered over the culet and girdle as well as parallel to the girdle. Otherwise, the stone may appear unbalanced due to the differing crown angles.

An off-center table may be listed as “Table Off-Center” (T/oc).

Misshapen Face

A proper diamond table should be octagonal, with 8 equally shaped sides that abut equally shaped facets. If the facets are not all the same size or shape or the table has uneven sides, it can alter the face-up appearance of the stone.

Misshapen facets that affect a diamond’s face-up appeal include bezels (MB) and stars (MS). If the star facets vary in length, it may be noted as a Star Percentage Variation (SPV). Or, the table may not be octagonal (T/oct or TEV).

Extra Facets

Sometimes, a gem cutter may add extra facets to a diamond, throwing off its sparkle. Although they can appear anywhere, extra facets typically manifest near the girdle on the pavilion.

You may notice an “Extra Facet” (EF) in your report, or maybe a facet is actually missing altogether! In this case, you may see “Missing Facet” (MF).

Inspecting a Diamond's Quality With a Loupe (10x Magnification)

Facets Not Pointing Upward

If a facet is not pointed, it interrupts the basic design of the diamond, which was engineered to shine as much as possible. For instance, a round brilliant diamond has 58 perfectly pointed facets aligned precisely for the ultimate sparkle.

A diamond may have a “Short Main” (SM) or “Short Bezel” (SB) if the point does not reach its intended location. But, it may also have an “Open Main” (OM) or “Open Bezel (OB) if the point is unfinished altogether. Or, if the pavilion has uneven points, it may be noted as a “Lower Half Variation” (LHV) in the paperwork.

Wavy Girdle

A diamond’s girdle should be parallel to the edges of its table, creating a straight, precise line. However, some diamonds have wavy girdles, where the edges waver slightly upward or downward.📈

Additionally, an imperfect girdle may be thicker in some spots than others, which can cause issues with light dispersion. “Girdle Thickness Variation” (GTV) would be noted on a diamond’s report.

Table and Girdle Not Parallel

Crown Height Variation in a DiamondIf the table and girdle are not parallel but the girdle is straight, the table may be the problem. The table may be off-center or tilted, which creates any uneven crown height. Gemologists may refer to this as a “Crown Height Variation” (CHV).

Crown and Pavilion Not Aligned

If the crown and pavilion are misaligned, the points of the diamond’s pavilion “mains” or upper facets do not meet perfectly with the lowest points of the bezel facets. These points should unite at the girdle to enhance a diamond’s symmetry.

A gemologist may note this imperfect as a “Misalignment” (Aln) on a report.

Naturals on Crown and Pavilion

Naturals are rough spots on a diamond from before the stone was polished. Usually, they start at the girdle and move either upward to the crown or downward into the pavilion.

If a diamond has rough patches, a gemologist will note it has “Natural” (N) sections.

Off-Center Culet

Diamond Symmetry Defect: Off-Center CuletThe bottom of a diamond may also be off-center, causing issues with refracted light. An easy way to see if a culet is off-center is to examine the diamond from the top, face-up view. If you stare into the table, the lower girdle facet lines should create a perfect square. If they bend in any direction, it is off-center.

If the culet deviates from the center position, gemologists will mark it as “Culet Off-Center” (C/oc).

Diamond fluorescence: A unique diamond effect

Another unique diamond characteristic is fluorescence. A fluorescent diamond glows when exposed to ultraviolet (UV) light. UV light sources included the sun and fluorescent lamps.

Minute impurities within diamonds cause fluorescence, and it occurs in 25% to 35% of colorless diamonds. 🔬Gemologists measure fluorescence on a scale of “None” to “Very Strong”, and the light emitted from the diamond may appear blue, yellow, or even orange.

Different Gradings of Diamond Fluorescence (None to Very Strong)

The perceived value of diamond fluorescence depends on the buyer. In some instances, blue fluorescence can make yellowish diamonds look slightly whiter, but overall the effects are negligible. Even trained professionals can’t always tell when a diamond will fluoresce without testing it! In the end, it’s all about whether you like fluorescent diamonds or are willing to test each stone under multiple types of light to ensure you like it from all angles.

Take the steps to understanding your diamond

A diamond’s cut is its most important quality, as it’s the most crucial factor in getting that characteristic diamond sparkle. To find a properly cut diamond, you should first understand a diamond’s anatomy and how each of these parts interacts with one another to disperse light through the stone to provide exceptional face-up appeal.

There are so many things to learn when shopping for a diamond, but we’re here to help. Next, we recommend checking out our article on the 4Cs for a deeper explanation of cut, color, clarity, and carat weight and how all of these factors affect a diamond’s value.