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Giving the gift of a diamond is the ultimate way to express your deepest feelings to the one you love. But with so much information and a range of price points to choose from, how can you be sure that the diamond you’re buying is the best it can be.

Diamond Buying Guide - The 4 C's

The value of a diamond is determined by four factors commonly known as The 4 C’s – Carat, Clarity, Colour and Cut. Before purchasing your diamond jewellery, it’s important to understand these factors as two different diamonds can look exactly the same, but be very different in value.



Referring to the weight of a diamond, and not the size, the term carat comes from the Mediterranean carob. The seeds of the tree were observed to be uniform in size and weight and used to balance scales when weighing precious gemstones for traders.

Just as £1 is divided into 100p, one carat is divided into 100 points. For example, a 50 point diamond weighs 0.50cts. But two diamonds of equal weight can have very different values depending on the other members of the 4C’s: clarity, colour and cut.

Precision is crucial when it comes to a diamonds weight as even a fraction of a carat can make a considerable difference in the cost. In the diamond industry, weight is often measured to the hundred thousandths of a carat, and rounded to a hundredth of a carat. Diamond weights which are greater than 1 carat are expressed in carats and decimals – for example, a 1.08ct stone would be described as ‘one point zero eight carats’.


Formed deep within the earth under extreme pressure and heat, diamonds often contain unique birthmarks - either internal (inclusions) or external (blemishes) – that can usually only be seen under professional magnification.

A diamond’s clarity refers to the absence of these inclusions and blemishes, with diamonds without these birthmarks being extremely rare and therefore the most valuable.

Using the GIA (Gemmological Institute of America) International Diamond Grading System™, diamonds are assigned a clarity grade that ranges from FL (flawless) to I3 (a diamond with obvious inclusions.

The GIA Clarity scale contains 11 grades, with most diamonds being graded as category VS (very slightly included) or SI (slightly included). The system takes into consideration the size, nature, position, colour and quantity of these clarity characteristics that are visible under 10 x magnification, to determine the grade of a diamond.




Diamond colour is all about what you can’t see. Valued by how closely they approach colourlessness – the less colour a diamond has, the higher its value – with most commercial diamonds being from colourless to near-colourless, with slight hints of yellow or brown. The exception to this, is fancy coloured diamonds, such as pinks and blues, which lie outside this colour range.

The GIA colour-grading scale for diamonds is the industry standard, with the scale beginning with the letter D (colourless) and continues with the increasing presence of colour to the letter Z (light yellow or brown). Each letter grade has a clearly defined range of colour appearance and are graded by comparing them to stones of known colour under controlled lights and precise viewing conditions. Many of these colour distinctions are so subtle that they are invisible to the untrained eye, but these minute differences make a very big difference in terms of quality and price.


The cut quality of a diamond is the factor that fuels a diamonds sparkle, fire and brilliance. The beauty of a diamond depends more on cut quality than any of the other 4 C’s.

The GIA Diamond Cut Grading System for standard brilliant cut diamonds in the D-Z colour range is based on the assessment of seven components:

o   Brightness: The total light reflected from a diamond

o   Fire: The dispersion of light into the colours of the spectrum

o   Scintillation: The pattern of light and dark areas and the flashes of light, or sparkle, when a diamond is moved

o   Weight ratio

o   Durability

o   Polish

o   Symmetry 

Each of these components is assessed individually, taking into account the relative importance of that component in the overall cut quality of the diamond, and the diamond is given a cut grade on a relative scale from Excellent to Poor, which represents a range of proportion sets and face-up appearances.

Diamond proportion refers to the relationship between the size, shape and angle of each facet and ultimately determines the diamonds interaction with light. When light strikes a diamond, approximately 20% is immediately reflected from the surface with the remaining 80% being reflected internally and then back out through the top of the stone. 

A well-proportioned diamond will have each facet placed and angled so it maximises the amount of light reflected and is perceived as the sparkle. If the diamond is cut too shallow or too deep, light will escape through the stone before it is reflected back through the top, causing the diamond to appear dull with less sparkle.