How to Measure Diamond Quality

How to Measure Diamond Quality

Diamonds hold an unmatched level of quality and beauty - which makes them the most popular choice for engagement rings and other precious jewellery. Diamonds contain a number of characteristics that determine their value. Potential buyers should be well-versed in how to measure a diamond’s quality to ensure they find their perfect stone. We’re all familiar with the saying ‘A diamond is forever’ - so it goes without saying you want to make sure you’re choosing a gem that lasts!

To help you out, we’ve put together this simple 5 step guide to measuring diamond quality as a non-professional.

  • Familiarise yourself with the 4C’s.
  • Test in different lights.
  • The Fog Test
  • The Transparency Test
  • The Water Test

1. Familiarise yourself with the 4C’s

The 4C’s are the global standard for assessing the quality of diamonds. They were created to allow you to compare one diamond to another and include Colour, Clarity, Cut and Carat.

Colour - Colour is measured using a system designed by the Gemiological Institute of America (GIA) and consists of a colour scale that ranges from D (colourless) to Z (light yellow or brownish in colour). The colour grade or hue of a diamond can have a significant effect on its value, as a colourless or near colourless diamond have the highest level of brilliance or sparkle.

Clarity - As a critical part of diamond grading, the level of visibility, the location, and the nature of internal (known as inclusions) and external (known as blemishes) imperfections are assessed to determine a diamond’s clarity. Because diamonds formed deep within the earth, under extreme heat and pressure, a diamond without these ‘birthmarks’ is exceptionally rare. The GIA scale involves 11 categories ranging from the extremely rare ‘Flawless Diamonds’ to diamonds with ‘Obvious Inclusions’.

Cut - The allure and brilliance of a diamond depends more on cut quality than anything else. To measure the cut quality of a diamond, it is largely based on how the gem interacts with light.  The diamond’s proportions, symmetry and polish are all considered when determining its brightness (how much light is reflected from the diamond), scintillation (the pattern of light or sparkle when the diamond is moved ) and fire (the dispersion of light into the colour spectrum e.g. any flashes of color).

Carat - Carat is the metric used to measure a diamond’s size and weight. One carat (not to be confused with ‘karat’, the metric for gold purity) is equivalent to 0.2 grams. Generally speaking, the greater a diamond’s weight, the greater a diamond’s value. However, two diamonds of equal weight can have very different values depending on the other members of the 4C’s: clarity, color and cut. 

Once you feel comfortable with using the 4C’s, have a think about which you are more likely to prioritise and if there are any that you are more likely to compromise on. This will help you simplify the process of sorting through diamond options and will help your jeweller to direct you to pieces that suit your quality specifications. Depending on where you are purchasing from, your diamond may come with an official Grading Report which you will now be able to understand.

2. Test your diamond in different lights.


A diamond can appear very different under various lighting conditions like daylight, fluorescent light and spot lighting. White spotlighting, which is commonly used in photagraphy brings out a diamond’s brilliance and fire, however, some diamonds may not always look as dazzling in less dramatic day-to-day light.

To measure the quality of a diamond, you should also observe the gem in average lighting conditions to get a real understanding of its light performance. 

3. The Fog Test

If you are not purchasing from a reputable jeweller or want to test the quality of a piece you already own, the fog test is a simple way to determine if your diamond is fake. As real diamonds can withstand heat very well, they will never fog up the way glass or other common diamond substitutes will. A quick and easy way to check is exhale on the stone like you’re trying to fog up a window on a cold day. If it does fog up, then it’s definitely glass and not a real diamond.

4. The Transparency Test


Another way you can test a diamond is with a simple transparency test. Take your loose diamond and place it on a newspaper, over a line of text.  A genuine diamond’s brilliance is likely to sparkle enough that it will be difficult to read the underlying print through the stone. 

However, if it is a diamond substitute like cubic zirconia, it will be clear and you will be able to easily read the text underneath the gemstone.

5. The Water Test

The water test can be used to determine the authenticity of a loose diamond based on density. Diamonds are known for their high density, ranging from 3150 to 3530 kilograms per cubic metre (over three times the density of water) in natural diamonds and 3520 kg/m3 in pure diamond. 

This means a diamond that isn’t set will sink to the bottom of the cup if placed in a glass of water as they are denser than water. However, fake diamonds like glass or quartz will float at the top or fall only to the middle of the glass.



In summary, understanding the 4C’s as well and knowing a few tests will give you a good chance at roughly pinpointing a diamond’s quality.  Now that you are familiar with how to measure a diamond’s quality as a purchaser, you may find it easier to shop for the best value diamonds in your price range. However, working with your jeweller to get a professional opinion is always optimal for an accurate benchmark on a diamond’s quality. 

We love working with diamonds and offer a unique Bespoke Diamond Engagement Ring Experience for those looking to explore a range of the highest quality diamonds while creating a phenomenal piece.

At Jason Ree, we are experienced jewellery experts with an eye for quality diamonds and precious gemstones. Explore our range of breathtaking ready-to-wear diamond engagement rings today or book a conversation with one of our experienced jewellery experts to discuss your dream piece.