Diamond 4 Cs

Colour – What colour is a diamond ? Blue, yellow, red, pink, brown, green, black ? It can be any of these; but I expect that the one in your ring is colourless. Even ‘colourless’ diamonds are graded into nine different shades, the finest being exceptional white, with cape or tinted colour (a yellowish tinge) as the lowest.

These differences are caused by the varying percentages of nitrogen in the stones, the best stones having the least nitrogen content. Most stones you see are ‘white’ or ‘commercial white’. However the colour  grading system currently in use is that developed by the Gemmological Institute of America (GIA). This uses letters from D through to Z, where D is the best. (It is unclear why it starts with ‘D’ rather than ‘A’.) Colour is judged by the use of a set of ‘master stones’ in a special light box. 

Clarity – This refers to the ‘cleanness’ or purity of the stone. Most gemstones contain small impurities, or inclusions; in diamond these are usually small pieces of black carbon, though they can contain other things. Lasers can be used to burn out carbon inclusions from a stone to improve its appearance. Grading is also subdivided into nine classifications, the top grade being pure or flawless. In fact this means that no inclusions are visible using a ten times magnification lens or loupe, though the stone may well have some inclusions visible under a microscope.

‘Pique’ means pricked, containing small inclusions usually visible to the naked eye. There have been several versions of colour and purity grading over the years and it varies from country to country, but this one on the right by the DIA is generally accepted at the moment.

As you can see, the distinctions between various degrees of colour and purity are open to a fair amount of expert judgement by the diamond grader and are not something that the layperson can judge for themselves. The tables are given purely as a guide to the meaning of the various terms used when describing stones.

Cut – This, as it sounds, simply describes the way that the stone is cut. For a diamond to display its full beauty and sparkle, the cut has to be correct and follow certain proportions. If a diamond is faceted at the wrong angles, which does sometimes occur to minimize wastage with an awkward piece of rough, it will not give the required dispersion and will affect the fully internal reflection necessary to allow the diamond to show its full fire. A shallow cut stone is termed a ‘fisheye’. Due to the diamond’s high refraction and dispersion, all light falling upon it, providing it is correctly cut, is reflected internally; no light should leak out of the back of the stone. If you look full onto the face of a correctly cut diamond, you cannot see through it. Ideal proportions differ, dependent on the refractive indices of the gem, so what is ideal for diamond is not, for instance, ideal for cubic zirconia.

This is only a small selection of the great number of cuts that have been used in the past and new styles are still being developed. Recently, new variations of the standard brilliant cut have been marketed; one of these is the ‘Eternal Cut’ – designed by Gabi Tolkowsky for Asprey & Garrard, it has 81 facets compared to the standard 57.A Japanese company claims to ‘change’ the colour of its diamonds by increasing the number of the facets to 86, making a deeper stone in the process. This alters the refraction of light through the gem, accentuating a particular spectral colour.All faceting of gemstones is carried out in similar manner, though diamonds are so much harder that they are handled slightly differently. Large diamonds are often cleaved: a hammer and chisel are used to spit the gem along natural cleavage planes within the stone. A skilled cutter can take days to decide where he will strike the stone; a wrong choice and the diamond will shatter. Once a large stone has been cleaved, it is handled in the same way as a small stone. First it is sawn to a rough shape, then ground into a double cone. Then the stone is passed to the polisher who cuts the facets onto the stone before polishing it. These last two operations are carried out on a lap-a rotating metal disc similar to a potter’s wheel – impregnated with diamond powder of decreasing fineness.In some countries gemstones (though not diamonds) are cut using a jamb peg: a pear-shaped block of wood covered with holes in calculated positions. The ‘peg’ or dopstick onto which the stone is attached with a special wax, is then inserted in the appropriate hole and the stone fed onto the rotating lap.Concave faceting is a startling new style of cutting, but availability is limited. Four examples are shown below, but there are many variations.

Mention should also be made here of the cabochon, which, though not used for diamond, is used for many translucent minerals or those showing particular characteristics such as ‘star stones’. It was fashionable at one time to cut sapphire in this manner. The cabochon is a dome, sometimes steep sometimes shallow, the name coming from the early French for ‘head’.

Carat – Diamond are weighed in carats, this is the recognized international standard. (In fact all gems are weighed in carats other than pearls, which come under a different classification and are weighed in grains.) A carat is 1/5 of a gram and is sub-divided into 100 points; there are 5 carats to a gram and 32 grams to a troy ounce.

So why carat ? It is believed to have originated from the use of the dried seeds of the ‘locust pod’ tree (Ceratonia siliqua) by early stone dealers to weigh stones due to the seed’s weight being so consistent-approximately 1/5 of a gram. The ‘metric carat’ was finally standardized in various countries in the late 19th and early 20th centuries. The carat weight of stones should not be confused with the karat purity of gold, although the terms are derived from the same source. Originally also called ‘carat’, karat with a ‘k’ was introduced in the USA to avoid confusion.

An approximate guide to the weight of a round diamond can be gauged by measuring the diameter of the stone. Table 2 lists weight against diameter in millimeters. It only applies to diamonds cut to near ideal proportions. It does not apply to other gems as their weight varies with their density.

All the 4 Cs should be taken into account when assessing a stone’s value and this can vary widely between the same size diamonds. A one-carat stone can be worth from a couple of hundred pounds to many thousands of pounds. Some coloured diamonds, termed ‘fancies’ by dealers, can rate £30,000 per carat. The rarest coloured diamond is red, followed by green, blue, pink and yellow. There is not a set rate per carat, however: smaller stones rate a low price per carat, the larger the stone the higher carat price.