What we look for when buying diamonds

Diamond Grading

Both natural and laboratory grown diamonds are graded to determine their quality and value. Every diamond is given a certificate detailing their grading report based on the 4 C’s. But what does this mean and what should you look out for when buying a diamond?


What are the 4 C’s?

When grading a diamond, gemmologists will look at the 4 C’s – colour, clarity, cut and carat weight. The diamond grading report will detail the assessment of the 4 C’s along with a plotted diagram of the diamonds clarity characteristics and a graphic representation of the diamonds proportions.


GIA (Gemmological Institute of America) changed the way diamonds were bought and sold in the 1940’s and 50’s by doing three things;


  1. Creating a common language for diamonds.


  1. Establishing the methods and best practices used to grade diamonds.


  1. Issuing the first GIA Diamond Grading Report, as a way to consistently and accurately communicate grading results


Virtually every diamond is described using the language GIA developed. Other reports from other entities have made their way into the marketplace, but GIA is recognised as the original and unrivalled source for accuracy and integrity in diamond grading. Expert gemmologists examine each diamond under controlled lighting and viewing conditions, in order to provide the most rigorous assessment of the 4 C’s . They are measured in the following ways.



Diamonds of gem quality vary in shade from colourless to yellow. To measure the colour, the diamonds are examined in a standardised viewing environment against a “master set”. Colour ranges from a D (colourless) to Z (yellow). A minimum of two graders must evaluate the stone to provide a colour assessment.



How a diamond had been cut, polished and to what proportions is extremely important since these determine the life, brilliance and lustre of the diamond. If the cutting factors fall below standard the diamond will be adversely affected. GIA provide a cut quality grade for standard or round brilliant cut diamonds on a scale of excellent to poor cut.



When forming, imperfections in the crystal structure can form and mineral impurities become trapped inside the stone. To assess the clarity of a diamond, gemologists will look at the number and nature of any inclusions in the stone as well as their size and position. Inclusions and blemishes are structural imperfections affecting the clarity of a diamond The analysis is carried out by a minimum of two graders under 10x magnification. Graders will them plot the clarity characteristics on a diagram best representing the diamonds shape.


The clarity grading scale ranges from –

Flawless – Internally Flawless – Very Very Slightly Included (VVS1 & VVS2) – Very Slightly Included (VS1 & VS2) – Slightly Included (SI1 & SI2) – Included (I1, I2 & I3)


It is incredibly rare to find a diamond without impurities. Generally, diamonds below a VS2 grade are likely to have inclusions which are visible to the eye.



The weight and size of a diamond is measured in carats (not to be confused with karats which signifies the purity of gold). One carat is 0.2 grams and there are 100 points (or 200 milligrams) per carat. Diamonds are weighed on an extremely accurate electronic micro-balance and the proportions, measurements and facet angles are determined by an optical measuring device. The size of a diamond increases with carat weight – however this is not a linear relationship but rather a curve. A 2.0ct diamond will not appear twice the size of a 1.0ct diamond as the distance across the top of the diamond must be considered. In fact the size difference on a round stone is only 1.35mm with the average measurement of a 0.50ct diamond being 5.00mm and the average size of a 1.0ct diamond being 6.35mm.


Expert tip 1

“When comparing similar diamonds try to look at the measurements. It is important to remember that diamond measurements are not standardised with carat weight. Some diamonds with similar weights can vary significantly in measurement – pick a diamond with wider measurements to gain a larger diamond surface for the same value.”


Diamond Fact 

The term carat originates from the Greek and Arabic names for the carob tree (Keration in Greek and Quirrat in Arabic). The dried seeds of the carob tree were widely used by trading merchants as counterweights when weighing gold, diamond, gemstones and pearls due to their relatively consistent weight and size.


In the Byzantine era, glass beads based on the seeds were used, however the use eventually diminished when it was discovered that despite the visual uniformity the seeds were not a consistent weight. In 1907 at the 4th general conference of weights and measurements the term carat was made the official metric measurement for gemstone weight.


Expert tip 2

“Certain diamond sizes are highly sought after and prices can vary dramatically depending on the carat weight. A 0.90ct diamond can be 10-20% better value than a 1.0ct diamond of the same quality but can appear almost identical in size but vary in price.”