Advanced Diamond Tutorial Part 2

If you are going to purchase a diamond, and know what you are getting when you purchase it, you need to be able to understand the information that is presented on a “lab grading report”, and how the information was obtained. Having this knowledge will further assist you to know if any follow up/independent data, presented to you by the jeweler, in conjunction with a “lab grading report”, is valid and accurate.

Here are several examples of “lab grading reports”. To gain a better understanding of how information is broken down on a lab report, we will examine one line by line.

NOTE: We will not be discussing the relevance of this information to the diamond’s cut quality, as that will be done in detail later on in this tutorial.

GIA Lab Grading Report

Figure 2.1

GIA Report

Courtesy of GIA

EGL USA Lab Grading Report

Figure 2.2

EGL USA Report

Courtesy of EGL

For the purposes of this tutorial, we will examine the AGS Lab Grading Report. Most reports have similar information on them and are simply arranged differently. Immediately below is the full copy of the “lab grading report”. Since the lettering on this report is quite small, we have included enlarged portions of the report below that will allow us to examine the information contained in this report more closely.


AGS Lab Grading Report

Figure 2.3

AGS Report

Courtesy of AGS

Figure 2.4

AGS Report

Courtesy of AGS

Figure 2.5

AGS Report

Courtesy of AGS

Figure 2.6

AGS Report

Courtesy of AGS

If we examine Figure 2.4, we see that this area of the report displays the basic information about the diamond. Let’s break this down point by point

Shape and Style

o   Simply stated, the shape and style of the stone…in this case Round Brilliant.


o   These are the measurements of the diamond in millimeters. The three numbers that make up the measurements given on the report are the 1) Maximum diameter of the stone, 2) Minimum diameter of the stone, 3) and the Depth of the stone. The diameter is the width of the stone as measured through the girdle. The Depth is the distance between the Table Facet and the Culet. NOTE: The average diameter measurements are very important, as most measurements expressed in percentages on a “lab grading report” are based on the average diameter measurement.

Cut Grade

o   This is the section that deals with the overall cut grade given by the laboratory grading the diamond. Different laboratories grade diamonds differently…these different grading methods will be discussed in detail later on in this tutorial.

o   On this report we can see the different categories…

§  Light Performance

·      This is the grade given to this diamond based on how it performed in the light reflection, brilliance, and other light performance analysis that it was subjected to during the grading process.

§  Proportion Factors   

·      In addition to testing diamonds for light performance, this laboratory also applies strict proportion standards to diamonds. It takes into account measurements such as depth, table, crown angle, pavilion angle, and minor facet measurements. The grade given to each diamond in this section is based on how well each of these measurements fit into the ranges prescribed by the grading laboratory.

§  Finish

·      Finish is comprised of two separate and distinct factors, polish and symmetry.

o   Polish

§  Polish is exactly what it sounds like…it is the polish work on the outside of the stone. After a cutter finishes the cutting and shaping of the diamond, he must polish the diamond to remove the marks made during the cutting of the stone, and give each facet a clear and highly reflective surface. This polish is graded on each facet, and only diamonds with exceptional polish across the whole diamond can receive the laboratory’s highest grade.

o   Symmetry

§  Symmetry is a factor that receives a lot of attention. There are different kinds of symmetry that pertain to a diamond, and it is important to understand each one, as they are completely different and affect the diamond in totally different ways. The symmetry that is graded on the lab report is the external symmetry of the diamond. It takes into account factors such as facet point alignment, table centering, facet alignment, consistency of facet shapes, etc.

·      Color Grade

o   This section deals with the color of the gem. Color is discussed in Emma Parker & Co.’s Diamond Tutorial. For questions about color, please refer to that tutorial.

·      Clarity Grade

o   This section deals with the color of the gem. Clarity is discussed in Emma Parker & Co.’s Diamond Tutorial. For questions about clarity, please refer to that tutorial.

·      Carat Weight

o   This section deals with the color of the gem. Carat weight is discussed in Emma Parker & Co.’s Diamond Tutorial. For questions about carat weight, please refer to that tutorial.


NOTE: A very important fact to understand about Color and Clarity is that the grading standards vary widely from on grading laboratory to another. Some of have much stricter standards than others, and this is something that should be carefully considered before purchasing a diamond.



Looking at Figure 2.5, we can see a proportion map of the diamond, with basic proportion and measurement data. The first thing that we notice when looking at this, is that the only two standards of measurements used are Percentages and Degrees. The very first measurement of the diamond is the 100%, which is equal to the average diameter of the stone at its widest point, the girdle. It is based off of this percentage that all of the other percentage measurements are calculated.


Directly under this measurement, we can see the 55.4% measurement. This is the diameter of the Table Facet, expressed in a percentage of the total average diameter.


The vertical line along the left had diamond is the measurement of the Total Depth of the diamond, again, expressed as a percentage of the total average diameter, in this case, 61.9%


The vertical line along the right side of the diamond, broken into three different measurements essentially breaks down the factors that make up the total depth of the diamond. These three factors are called Crown Height, Girdle Thickness, and Pavilion Depth.  In this case you can see the three measurements are as follows.


·      Crown Height – 15.5%

·      Girdle Thickness – 1.6% to 4.4%

·      Pavilion Depth – 42.8%


The two angle measurements on the left hand side of the graph are the Crown Angle and Pavilion Angle measurements. In this case, the Crown Angle is 34.8 degrees, and the Pavilion angle is 40.6 degrees.


The two numbers in the diamond represent facet length measurements.  The measurement in the small facets on top is the Star Facet Length, and the measurement in the slightly larger facets on the bottom is the Lower Girdle Facet Length. For this diamond, the Star Facets measure 51% and the Lower Girdle Facet’s measure 75%.


It is very important to understand that these two percentages are not calculated against the total depth. Rather Star Facet Lengths are expressed as a percentage of the total distance between the edge of the Table and the edge of the Girdle. Lower Girdle Facet Lengths are expressed in a percentage of the total distance between the Culet and the edge of the Girdle. (See illustration below)



Star Fact Measurement

The Green arrow, in Figure 2.7, represents 100% of the distance from the edge of the TABLE to the edge of the GIRDLE. The STAR FACET measurement is expressed as a percentage of the total measurement.

Figure 2.7

Graph showing diamond girdle height

Copyright Emma Parker & Co. 2008

Lower Girdle Facet Measurement

The green arrow in Figure 2.8 represents 100% of the distance between the edge of the GIRDLE and the CULET. The LOWER GIRDLE FACET measurement is expressed as a percentage of the total measurement.


Figure 2.8

Graph showing diamond pavilion depth

Copyright Emma Parker & Co. 2008

Lastly at the bottom, we see the Culet Size. This is almost always expressed as a word. Culet sizes vary including sizes of None, Pointed, Very Small, Small, Medium, etc.

The last graphic we see, Figure 2.6, is the Clarity Plot of the diamond. This is where the grading inclusions, that were visible at 10x magnification, are plotted by the grader. Clarity has already been discussed in brief in the Emma Parker & Co. Diamond Tutorial, however there are some simple hints to help you spot important information on the Clarity Plot. All inclusions plotted in RED INK are INTERNAL inclusions. All inclusions plotted in GREEN INK are EXTERNAL inclusions.

This basic walk through of the information that is listed on a lab grading report will give you a good foundation in what information you will be presented with later on in this tutorial, and what each measurement relates to. In the following chapters we are going to dig deeper into cut, what to look for in a great cut, what tools and reports are useful in looking for a good cut and how to read them, and more.