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Diamond Grading

Carat, Colour, Clarity and Cut

As diamond investment grew, it became essential to have a grading system which would be recognised throughout the world. The Gemological Institute of America (GIA) introduced the International Diamond Grading System and the 4Cs to compare and evaluate diamonds. The 4Cs are the Carat, Colour, Clarity and the Cut of a diamond – all of which affect its value.

Carat

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Not to be confused with the karat, which is used for measuring the purity of gold, the carat rating for diamonds and other gemstones refers to its weight. Early gem traders would use the seeds from the carob plant to counter their scales, which is where the term carat originated. Established in 1918, the modern carat system unified the weighing system making one carat equal to 0.2 grams. Carats are divided into 100 points, for example a 20-point diamond weighs 0.20 carats.

gia carat

The majority of diamonds used in fine jewellery weigh one carat or less. A fraction of a carat can make a considerable difference in cost. When weighing stones precision is crucial and weight is often measured to the hundred thousandth of a carat, and then rounded to a hundredth of a carat.

Stones which weigh more than one carat are expressed in carats and decimals. However, even two diamonds of the same weight can have very different values depending on the other three C’s.

Colour

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Before the GIA introduced the Colour Grading Scale, a number of other systems were used to grade diamonds. These used alphabetical (A B & C) and numerical (both Roman and Arabic) values and the result was inconsistency and inaccuracy. Because the GIA required a unique system, they chose to start their grading system with the letter D, never previously used. This has become the international industry standard for measuring a diamond’s colour.

Colours d to z

The clearer a diamond is, the higher its value. Most diamonds used in jewellery run from colourless to near – colourless, with slight hints of yellow or brown. For example, a colourless diamond will be given either a D, E or F grade whereas a tinted (usually yellow or brownish) diamond, where the colour is visible to the naked eye, would be given an S – Z rating.

Diamonds which fall between these colour ranges are given grades G (nearly colourless diamond) through to R (lightly tinted, usually yellow). Although at the lower end of the colour scale, colour is visible to the untrained eye, many diamonds express very subtle colour distinctions only visible when viewed by a professional – a slight difference in colour can mean a big difference in the price of a diamond.

Naturally fancy coloured diamonds have their own grading system taking into account their colour, saturation, hue and tone.

Clarity

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Diamonds often contain unique marks either inside the diamond itself; an inclusion, or on the surface of the stone. These blemishes are caused in the formation of the diamond under extreme heat and pressure deep within the earth. They can also make the diamond more likely to shatter.The clarity of a diamond refers to the absence of these inclusions and blemishes and as diamonds without any marks are rare, they are therefore more valuable.

The GIA system assigns diamonds with a clarity grade which ranges from flawless (FL) to diamonds with very obvious inclusions or blemishes (I3). A flawless diamond will show no internal or external imperfections whereas a VVS1 or VVS2 (very, very, slightly included) diamond will have no internal flaws but will have slight external blemishes – often only visible under a jeweller’s loupe.

Clarity

Most categories of clarity will not affect the stone’s brilliance and it isn’t until you reach the bottom end of the scale – the I1, I2 and I3 included diamonds that the imperfections will begin to detract from the beauty of the diamond.

Although there are a total of 11 grades of clarity, most diamonds fall into the VS (very slightly included) or the SI (slightly included) grades. Completely flawless diamonds are extremely rare.

Cut

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Of all the 4Cs, the cut of a diamond has the greatest effect on its beauty. Diamonds have a unique ability to manipulate light and this can be released and maximized only by cutting and polishing the diamond to a high level of accuracy. When a diamond is cut well it refracts and reflects light to the maximum degree.Similarly, you may have a colourless, flawless diamond but if it is not cut well it will appear dull and lifeless. Firstly, the shape of a diamond is established – the most common is a round brilliant stone and all others are known as fancy shapes. Traditional fancy shapes include the marquise, pear, oval and emerald cuts although different shapes such as hearts, triangles and squares are gaining popularity.

Anatomy of a BR cut

Difficult to analyse or quantify, the cut of a diamond can be split into three different attributes; brilliance (the light reflected from a diamond); fire (the dispersion of light into the colours of the spectrum) and scintillation (how a diamond sparkles when moved).

When looking at the cut of a diamond to express its value, various aspects are taken into consideration – the diamond’s proportions, its symmetry and the polish.

The depth of a diamond, called the Pavilion, can greatly affect the diamond’s brilliance and cutters have to be experts in balancing the retention of the stone’s weight against a pavilion depth which, if too shallow or too deep, will allow light to escape through the sides or bottom of the diamond. A well cut diamond will direct more light through the crown of the stone thus making it more brilliant to the eye.

The number of cuts is also important – a standard round brilliant diamond has 57 or 58 facets (the 58th being the flat facet at the bottom of the pavilion known as the culet). The large flat facet on the top is called the table. The proportions of a diamond refer to the relationships between the size of the table, angle of the crown and depth of pavilion.

After 15 years of meticulous research the GIA launched its diamond cut grading system in 2005 for standard round brilliant diamonds in the D – Z colour range.

Taking all of the above into account, diamonds are awarded a cut grade of Excellent, Very Good, Good, Fair or Poor.

In addition to the 4Cs, a diamond graded by the GIA also gives information regarding the make-up of an individual diamond. It is these factors, combined with the 4Cs that determines the value.

They include:
  • Date of Grading
  • Report Number
  • Laser Inscription
  • Registry
  • Shape and Cutting Style
  • Measurements
  • Finish
  • Polish
  • Symmetry
  • Fluorescence
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Andrew Segal
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