The inherent ‘fire’ and beauty of a diamond is only unleashed and observed after it has been cleverly crafted by a diamond cutter who has to polish tiny facets into the surface of the pebble-like rough gem stone. It is only then that jewellers and gemmologists can assess and grade polished diamonds using the 4 C’s classification system – cut, carat, colour and clarity.
The superlative optical properties of the polished diamond are maximised by the exact position and angles of the facets in relation to each other. A well cut diamond will produce greater brilliance and so laboratory-devised guidelines determine the grade of cut.
Cut also refers to shape; round diamonds or fancy shapes. The symmetrical round diamond is the most brilliant of all the shapes but guidelines are also given to determine the grade of cut on fancy shapes.
The colour of gemstones is the first major characteristic feature to be recognised by the human eye. Colour is produced by the absorption of white light, which is made up of the spectral / rainbow colours. Each individual colour component is absorbed at various rates dependent on the chemical and atomic structure of the mineral.
Pure diamonds appear colourless because almost all the wavelengths of the spectrum are equally strongly absorbed. Trace elements present in diamonds and irregularities in their molecular structure will produce rare fancy coloured diamonds. Minute traces of boron in the atomic structure will produce fancy blues, and nitrogen fancy yellows. Natural pink diamonds owe their colour to the deformation of the molecular structure due to geological processes after the diamond’s formation.
The white diamonds are graded on a colour scale ranging from the top and very rare D (colourless) to Z. From D, the subtle saturation of yellow hues becomes more obvious down the scale. Top white diamonds D – H are the most sought after but it is often difficult to identify the differences between two colours without strict lighting conditions and a trained eye.
As well as colour, clarity and cut, weight provides an additional basis in the valuation of a diamond. The weight of a diamond, and all gemstones, is measured in carats, commercially abbreviated to ‘ct’.
In metric terms 1ct = 200mg, or 1gramm = 5cts.
Historically, the term carat derives from the word carob – from carob seeds, when they were used as a reference for diamond weight. In these ancient times, one carob equalled one carat.
The formation of diamonds, deep within the earth’s interior, occurs under variable pressure, temperature and cooling conditions. Fracture marks, tension cracks, clouds and even the presence of enclosed minerals are the fingerprints of these variable conditions.
We refer to them as inclusions and they are used as identifying characteristics with which to grade the clarity of the diamond. A scale of clarity ranges from the rarer F and IF (flawless and internally flawless) to the visible I (included), and is based on the visibility of inclusions at a magnification of x10.