How Diamonds Are Graded?

We understand that you might be afraid of being scammed when buying a diamond for the first time. It’s important to understand the grading of diamonds so you know if you are getting a good price.

Diamonds are graded by the American Gem Society, the Gemological Institute of America, and other independent labs. They grade diamonds by assessing the cut, color, carat, and clarity of each gem. Each lab uses a different scale for grading diamonds.

In this article, we will help you understand who gives the grades and how they are given. We cover this in detail so you can make informed choices.

How do labs grade diamonds?

Diamonds are graded by gemological labs. The more consistent the grades are, the more reliable the lab is. They grade diamonds so they can find out how valuable they are and so put a price on them.

There are different criteria that labs use to grade diamonds, but the most common are cut, color, and clarity.

Cut refers to the cut of the diamond facets, its level of polishing, symmetry, and depth. This is straightforward and often the same grading between different labs.

Clarity refers to how perfect the diamond is. The fewer imperfections a diamond has, the greater the clarity. The color of a diamond is decided by its amount of yellow tint.

These criteria are not as objective as cut or carat. So, two labs may give different clarity and color grades for the same diamond.

That is the reason why many jewelry stores send diamonds to be graded by different entities. If one gives them a lower score, they send it to the other to get a better price.

There are two prominent lab grades to know: the American Gem Society (AGS) and the Gemological Institute of America (GIA).

Their grades are reliable because the labs are consistent. If you are buying online, stick with GIA or AGS.

The AGS Grade

The AGS was the first lab to grade diamonds in North America. For a long time, they were also the only lab.

The GIA has since taken half of the market, but the AGS is still the most used grading system for big brands like Blue Nile and James Allen.

The grade used by the American Gem Society is simple and easy to understand. They grade diamonds on each criterion and assign a value from one to ten.

If a diamond is perfect, it gets a zero grading. Ten is the worst possible grade.

The system uses three criteria:

  • Cut
  • Color
  • Clarity

This means that if a diamond has some yellowish color it gets a grade from 1 to 10 in color. It is still valuable but will be worth less because of its tint.

If a diamond is perfectly polished and has flaws, then it gets a zero in clarity. This is a perfect score.

However, you should know that AGS criteria is generally used on round diamonds.

So, you should be careful of their grading of oddly shaped diamonds. It’s better to verify this grading with other labs.

The GIA Grade

The Gemological Institute of America is known to be the most reliable and distinguished laboratory in the United States and the world.

Their grade works similarly to the AGS. However, they also consider carat in their grading system.

Carat is decided by weight. So, heavier diamonds are more expensive.

However, the increase in price is not proportionate. Because larger diamonds are rarer, a one-carat diamond could be 2-3 times the price of a 0.5-carat diamond.

The clarity grade from GIA has eleven categories. The most perfect score in the criteria would be flawless. The least favorable score is I3, which means that the diamond has visible flaws.

Color is graded from D to Z. D is the best score and Z is the worst. For a diamond to get a Z score, it means that it has a significant yellow tint. However, it is still valuable.

The GIA cut grade has five possible scores:

  1. Excellent: When a diamond has the best proportions possible.
  2. Very Good: When there are flaws that are unnoticeable to the naked eye.
  3. Good: When proportions are still on point but with certain visible flaws.
  4. Fair: When the diamond has the right proportions to the eye, but any deep inspection would reveal the flaws.
  5. Poor: When a diamond is disproportionate.

These four criteria (cut, color, clarity, and carat) are called the four C’s in the industry.


Diamonds are graded by labs. Each lab has a different grading system, and they are considered reliable when their grades are consistent.

The GIA and the AGS are the most reliable labs. Both use a grading system that considers cut, color, and clarity. The GIA also uses carat to price their diamonds.

There are some independent labs that use additional or different criteria. However, they are not as reputable and so you should take their grading with a pinch of salt.


Carl A. Jones, GIA GG

Carl A. Jones is a GIA Graduate Gemologist with over 20 years of experience in the diamond industry. He is an independent jewelry appraiser. He specializes in determining the value of diamonds and advising consumers on how to buy quality diamond jewelry.

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