Sotheby’s Gem and Jewellery

by Brett O’Connor


I have been collecting gems since I was a child and for the past quarter century I have made my living by studying them, so when I am shown a gem certificate, I instinctively know what to look for.

The other day I was sitting with a husband and wife who were showing me a diamond they had inherited. They were thinking of selling it and in preparation had obtained a GIA (Gemological Institute of America) report. When I looked at the certificate, there were many issues with the stone but the clients believed that because it was high colour and high clarity, the value must be high. After explaining the certificate to them and how it affected their stone, I thought there must be many people in the same situation either selling or possibly buying a diamond who could use some help in deciphering the information contained in the certificate. The following information can help you to understand what the certificate means to your diamond.

There are many good gemmologists and gem labs out there but the GIA is really the premier lab when it comes to diamonds. In 1938 it created the modern diamond grading system which has been adopted by the rest of the world and is considered the standard for all other diamond certificates. They have rightly gained a reputation for consistency and I will refer to their certificates in this article.

Basic Identifying Details

If you don’t have a certificate to look at, you can use the image above for reference.

Starting at the top left side you will find comprehensive information which is unique to your stone. It starts with the date the certificate was produced. Typically if a cert is over 5 years old, you will want to request an updated one. The date becomes very important if the stone you are buying is high colour and clarity – if the stone has an Internally Flawless or better clarity then I would suggest a regrade for any cert over 1 year old. This provides piece of mind that the stone you are buying is still graded as stated on the report. Then the certificate number is listed. You can enter this number into the GIA website and it will verify that the report is authentic- alternatively you can scan the QR code in the lower right hand corner with your smartphone to obtain the same information.

Below the certificate number are the details that were commonly known as the 4 C’s. On the certificate they are referred to as grading results as there are now more than 4.

Cut – This is the shape of your diamond. It could be round, pear shape, cushion, rectangular, marquise, oval or heart shape. It also shows the measurements in millimetres.

Carat weight – This is listed to the hundredth of a carat.

Colour grade – These range from D (colourless) to Z (very noticeable colour). The most common colour is yellow. This is caused by nitrogen impurities in the crystal structure.

Clarity grade – Starting with Flawless down to I3 (imperfect) Starting with SI1 you can begin to see some inclusions with the unaided eye.

Cut grade – This is the newest addition to the group. It takes the measurements, polish and symmetry into account and is given a grade based upon these factors. Excellent is the most desired and ranges down to poor.

There are 3 grading scales on the right side of the certificate, which provide a visual guide for you to use.

Additional Grading Information

Polish and symmetry grade how well the diamond cutter did his or her job. They use the same scale as the Cut scale located on the right of the certificate. Excellent down to poor.

Fluorescence – This is a naturally occurring phenomenon in which some stones emit a colour (typically blue) when exposed to ultraviolet light. This is an important characteristic as it can affect the brilliance of a diamond and have a large impact on price. The more fluorescence the stone emits, the lower the value of the stone. The most desirable grading is none. It then progresses to faint, moderate, and then strong. Just remember that at moderate and above, the fluorescence will usually have a detrimental effect on the value of the stone.

The Comments Section

Ideally you would like a cert to be free of comments but some acceptable and frequent ones might be “additional clouds not shown” or “pinpoints are not shown”. The one which could affect the price in a negative way would be any phrase with the word graining in it. Graining is a naturally occurring characteristic which can reduce the brilliance of the stone depending on how much the stone has. Frankly, it is best avoided.

In the center column of the certificate is a profile diagram which illustrates the proportions of the diamond. These can be confusing as there are many percentages and angles listed.

The ones to focus on are listed below:

Table percentage – This is the number in the center of the diagram and is the width of the top of the diamond (the table or largest facet), as a percentage of the total width of the diamond. If the table is too big or too small, your brilliance will be adversely affected. Ideally a round diamond should be within the range of 53% and 64%

Depth percentage – This is a percentage, shown at the top center of the diamond diagram that represents the depth of the diamond compared to the width. Again, if it is too deep or too shallow, it can affect the brilliance of the stone. Ideally a round diamond should be within the range of 58% and 64%

Culet – The culet is the very bottom or point of a round diamond. Although it is a personal preference as some old cut stones have large culets which many people find charming, I would suggest that in general one would choose either none or very small.

Girdle – located on the profile on the left hand side, this is the thickness of the waist or widest part of the diamond. The girdle is likely to have some variation but typically we like to see thin to slightly thick. Where it starts to become undesirable is when it falls on either side of this range… very thin or very thick.

The Final Section is the clarity characteristics

This is where the team at GIA who grade the diamond actually plot on the diagram the size, location and type of inclusion on or within your diamond. It is a unique map of your diamond’s clarity characteristics. There are many types and they can be black, brown, grey or even colourless but ultimately the less of them there are the more desirable the diamond. The clarity grade is based upon this plot.

Remember that each diamond is unique, like a work of art from Mother Nature’s studio and so the information provided in these certificates can be interpreted in slightly different ways which can alter the value of your diamond. I hope this basic information will allow you to feel more confident the next time you pick up a GIA certificate when thinking of buying or selling a diamond.

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Brett O'Connor
About Brett O'Connor 5 Articles
Brett O’Connor joined Sotheby’s in 1999 and has been based in Geneva since 2002, sourcing gems for Sotheby’s worldwide Jewellery auctions. He has over 22 years of auction experience on three continents and has been involved with many of the important jewellery collections, including the collections of Baroness Eugene de Rothchild, Luisa Fanti Melloni, Maria Callas, Madame Claude Arpels, Lily Marinho, Count Guido Henkle von Donnermarck and Gina Lollobrigida. Mr O’Connor earned a Graduate Gemologist diploma from the Gemological Institute of America in Santa Monica, California, in 1994.

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