The Diamond Pro

AGS Certification: Everything to Know Before Buying a Diamond

By Michael Fried

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Bottom Line Recommendation:

The AGS has established itself as a top-tier diamond grading organization. As one of the most reliable and reputable lab grading entities, we recommend buyers seek an AGS certified diamond (or a GIA certified diamond). When choosing an AGS diamond, be sure to buy from a high-quality diamond vendor, like Blue Nile or James Allen.

History of AGS

In 1934, a small group of jewelers who wanted to protect buyers from false advertising and fraud in the diamond industry established the American Gem Society (AGS). They were the first organization to release a diamond cut grading system when they did so in 1966.

Today, the AGS provides retailers, jewelers and individuals with gemological knowledge, research, consumer protection and standard grading of diamonds. With clear and consistent grading, the AGS is one of the most trusted lab grading entities in the world.

AGS Diamond Grading Report

As a non-profit independent grading entity, the AGS has no stake in the sale of a diamond. The AGS evaluates each diamond based on its qualities and components.

Grading Results and Scale

A diamond’s 4 C’s (Cut, Color, Clarity and Carat) represent the four main components of its structure and beauty. AGS grades each C on a scale—helping to determine the value of a diamond and its level of quality.


Diamond Cut grade indicates the quality of a diamond’s angles, proportions, symmetrical facets, brilliance, fire, scintillation and finishing details.
Cut is graded differently by the AGS than other labs like the GIA. The AGS assigns a number along with a descriptive word grading.

The AGS Cut Scale is as follows from 0 to 9, 0 being Ideal and 10 meaning Poor:

AGS diamond cut scale

The Ideal and Excellent grades, depending on Shape, demonstrate proportions and angles cut for maximum fire and brilliance.

How Light reflects through a diamond

It’s important to know that a top designation, like Excellent, doesn’t always indicate an outstanding diamond. Almost 55% of diamonds sold online are graded as Excellent cuts. Some are mediocre, while others are stunning like this 1.5 Carat Round Brilliant from James Allen.

Because Cut is critical to a diamond’s beauty, it’s necessary to review Cut quality and ask for an expert’s opinion.


Color grade determines how white or colorless a diamond is. AGS grades diamonds on the following Color scale, from 0 to 10 (with 0 being the most colorless and 10 denoting noticeable yellow or brown tint).

The chart below shows how the AGS and GIA color grades overlap with each other. If you are comparing two diamonds, one certified by GIA and one by AGS, you’ll be able to compare their colors using this chart.

AGS color scale

In almost all cases, the naked eye is unable to distinguish two adjacently color graded diamonds, although the price difference may be significant. The color grade given by AGS helps identify the quality of the diamond’s Color, yet it’s still important to determine if the diamond appears colorless to your own eye without magnification.

Note: Fancy Diamonds, like a purple or blue diamond, are valued gemstones.

Their color grades are different than those of traditional “white” diamonds.


Similar to Cut and Color, AGS grades Clarity on a 10-point scale. Clarity grade refers to how clean a diamond is from inclusions and blemishes.

AGS clarity scale

For Clarity, we recommend looking for a diamond that is eye clean. If you’re unsure how to evaluate a diamond for Clarity or any other characteristic, contact one of our experts.


The weight of a diamond, not its size, is what Carat refers to. For perspective, a 1 Carat Diamond equals 200 milligrams or 0.2 grams—meaning it weighs about the same as a quarter of a raisin. Two 1 Carat Diamonds can be quite different in size, depending on Diamond Shape and how the gemstone is cut.

The overall beauty of a diamond should carry more importance than Carat weight when deciding which diamond to purchase.


In addition to the 4 C’s, an AGS report provides information about the width, depth and table size of a diamond.

Diamond Table Percentage

To calculate the diamond table %, the width of the table (top surface area) is divided by its width (diameter). If the table facet is 3.5 mm wide, and the diamond is 5 mm wide, the table % is 70%.

An ideal table % depends on the diamond’s shape. To determine if a diamond has a high-quality table %, contact us and we’ll be happy to assist.

Diamond Width

A diamond’s width is measured as the length from one end of its girdle (the diameter at its widest point) to the other end of the girdle. Width is important when it comes to calculating a diamond’s length to width ratio, which determines how proportionate the stone is.

Depth Percentage

The height of the diamond from the culet to the top of the table is considered the depth %. The depth % is calculated by dividing the depth by the width. This measurement is reported in millimeters and percentage. For instance, a diamond of 4mm in depth and 4.5 mm in width would have a depth percentage of 88.8%.

A lower depth % of two equal carat diamonds usually appears larger due to increased width. A depth % that is too low, though, can create a dark appearance as it will not reflect light very well.

Additional Grading Information

In addition to the 4 C’s and proportions, an AGS report includes further details, like finish, polish, symmetry and fluorescence.

These aspects impact a diamond’s beauty, but they are not the most important components to consider when making a decision.

How AGS Certification Impacts Cost

The fact that a diamond has or doesn’t have an AGS certification does not make the gemstone more expensive. The AGS is an independent grading entity and does not have a stake in the sale of a diamond. Diamonds certified by the AGS sell at similar prices to their GIA equivalents.

Upgrade Shopping

The one issue to look out for is “upgrade shopping”. No lab is 100% accurate as color and clarity are not objective grades (like weight and dimensions are). If there is a diamond that is somewhere in between grades, a manufacturer or retailer may send the diamond to multiple laboratories looking for the better grade. For example, say a diamond is a weak I color or strong J color, and they received a J color from GIA. The wholesaler/retailer may try sending it to AGS for the I color (they can then sell it for much more money). Its far more likely that the grades will be the same, but its possible they will get the upgrade (which more than makes up for the cost of sending it to AGS for certification).

It is very common for companies to use AGS for branded super ideal diamonds (like True Hearts, or Hearts on Fire). But AGS is not as commonly used for non-round diamonds. So if a retailer has 50 cushion cut diamonds, 49 of which are GIA certified and one AGS diamond, that diamond is likely to have received an upgrade.

This is not a knock on AGS, merely an observation of how some wholesalers and retailers may try to take advantage of the end consumer.

Advantages of AGS Certification

  • Highly respected certification
  • Offers consistent and reliable grading
  • Ensures you’re receiving the diamond quality that the certificate states
  • Provides six security measures to help ensure authenticity

Disadvantages of AGS Certification

  • Be wary of upgrade shopping

Unique Reports and Additional Services

In addition to evaluating diamonds, the AGS provides:

We strongly recommend looking for a diamond that is AGS or GIA Certified. We also encourage purchasing a diamond only from a reputable vendor like Blue Nile or James Allen.

For assistance in reading a report or comparing diamonds, feel welcome to contact our experts.

About the author
Mike learned the diamond business from the ground-up at Leo Schachter Diamonds - one of the world's top diamond manufacturers. He has been recognized as a diamond industry expert by Time, PeopleMoney, The Daily Mirror, NerdWallet, The Times Herald, Yahoo Finance Australia, The Art of Charm, The Washington Diplomat, The Next Web, and more. See more
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