How to choose a diamond



Before the advent of lab-grown diamonds (the quality of which can generally be assured), buyers faced a far more difficult challenge when judging a diamond’s value. Today, customers can select a Type IIa cultivated diamond with confidence, safe in the knowledge that it will have excellent clarity and a precise, sparkling cut. However, the value of diamonds – whether cultivated or mined – still varies according to four key factors and it is perhaps worth examining how these factors are judged.

The “Four Cs”

Naturally occurring diamonds vary considerably in purity and colour so, in the last century, the international diamond trading community agreed upon a standard means of classification. This is based on what are colloquially known as the “four Cs” – carat, colour, clarity and cut.


Carat is simply a measurement of weight and, all else being equal, a larger, heavier diamond will be more valuable than a smaller, lighter one. A metric carat (usually abbreviated to ‘ct’) is equivalent to 200 milligrams. For the purposes of accuracy, weights may be expressed in carats to two decimal places. For example, the famous ‘Elizabeth Taylor Diamond’ is described as a 33.19 carat Type IIa diamond. Note that the abbreviation ‘ctw’ (carat total weight) is also sometimes used.


A pure carbon diamond with a perfect lattice structure – or in other words, the ‘perfect diamond’ – would be completely clear. The colour within a diamond is normally due to impurities or imperfections in its structure. Most commonly, it is caused by the presence of nitrogen atoms, which might be evenly spread or grouped into clusters. The arrangement of nitrogen within the molecular lattice of the diamond will have a great bearing on its colour, which might vary from yellow or brown to orange or even a yellowish green. Diamonds that include nitrogen represent about 98% of all mined diamonds and, as such, they are termed ‘Type I’ diamonds.

Type II diamonds contain no nitrogen. Type IIb gems may, however, contain boron, which imparts a grey or blue tint. Type IIa – the very purest type – contain virtually no impurities at all. In such cases, the only colour variation tends to result from slight deformities in the lattice structure, and these can result in a pink or reddish hue. Colour has a significant bearing on the price of a diamond. Generally, the closer it is to absolutely colourless, the greater its value. An alphabetic scale is used, with ‘A’ denoting a completely colourless stone, and ‘Z’ representing one with considerable colouration. However, an important exception is the category of ‘fancy’ diamonds, which have a colour more intense than a Z rating. Much prized for jewellery, highly coloured diamonds of this sort will tend to fetch a higher price than one with a pale hue.


Clarity is a self-explanatory measure of how free a diamond is from internal flaws or surface imperfections. The ideal is a completely flawless gem and stones of this kind are exceptionally rare. Most inclusions are invisible to the naked eye but a cluster of them may render the diamond ‘cloudy’. The presence of tiny cracks can also produce a white or cloudy appearance, and such imperfections might also limit the diamond’s propensity to scatter light. The Gemological Institute of America has set out a scale that rates clarity as follows:

FLFlawlessthe rarest and most valuable type of diamond
IFInternally Flawlessvery slight blemishes on the surface only
VVS1Very Very Slightly Includedthese exhibit tiny flaws that even an experienced assessor will find difficult to spot under 10x magnification
VVS2Very Very Slightly Includedas above, with slightly diminished clarity
VS1 & 2Very Slightly Includedtwo similar grades: these denote the presence of minor imperfections that a trained assessor will find easier to see under 10x magnification
SI1 & 2Slightly Includedtwo similar grades: these denote the presence of more obvious inclusions that a trained assessor will find easy to see under 10x magnification; some flaws may be apparent to the naked eye
I1, 2 and 3Includedthree similar grades: these denote the presence of obvious inclusions that a trained assessor will find easy to see under 10x magnification; many will be visible without magnification. I3 diamonds will have large, obvious inclusions that may reduce the stone’s brilliance or even threaten its structural integrity

Note that other international organisations have adopted alternative scales but the GIA grading is arguably the most widely accepted.


A diamond’s cut should not be confused with its shape. Shape is simply its three dimensional appearance and various terms are used to describe popular shapes such as cushion, round, princess, oval, pear or heart. The shape itself will not necessarily have a marked bearing on a diamond’s value. ‘Cut’, however, refers to the quality of the workmanship – the symmetry of the facets and the way the diamond has been made to refract light. A diamond’s cut can make a significant difference to how light plays on and through it, so it is considered a very important criterion when judging a diamond’s value.

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