The 4Cs of a diamond: how to choose a diamond

The 4Cs give every diamond a common language, so two stones can be compared on paper before you see them. GIA introduced the system in the 1950s, and a GIA report is still the most widely recognised way to describe a diamond.

No single C makes a diamond beautiful. A large stone with a poor cut looks dull, while a well-cut stone of slightly lower colour can look brighter than one with a better grade. The skill lies in deciding where to spend and where to save.

Carat is a measure of weight, not size: one carat equals 0.2 grams. Price rises faster than weight, because large rough diamonds are rarer, so a two-carat stone costs far more than two one-carat stones of the same quality.

Prices also jump at round weights such as 1.00, 1.50 and 2.00 carats. A stone of 1.45 carats looks almost the same as one of 1.50 but is usually priced noticeably lower. In our collection, a round solitaire of 1.52 carats is priced at €20,900 and a round ring of 2.01 carats with marquise side stones at €36,500.

Colour is graded on a scale from D, completely colourless, to Z, with a light yellow or brown tint. D to F are called colourless and G to J near colourless; the difference between neighbouring grades is hard to see with the naked eye.

The metal of the ring affects how colour reads. Platinum and white gold show any warmth in the stone, so F to G is a sensible choice there. Yellow and rose gold reflect warm tones, which lets a slightly lower colour grade look whiter.

Clarity describes the tiny inclusions inside a diamond and the blemishes on its surface. The GIA scale runs from Flawless (FL) and Internally Flawless (IF) through VVS1 and VVS2, VS1 and VS2, SI1 and SI2, down to I1, I2 and I3.

What matters most is whether a stone is eye-clean, meaning you see no inclusions without magnification. Many VS2 and some SI1 stones are eye-clean in a round brilliant. Step cuts such as the emerald cut have large open facets that show inclusions more clearly, so they call for a higher clarity grade.

Cut decides how a diamond returns light, and it is the C with the greatest effect on sparkle. For round brilliants GIA grades cut from Excellent to Poor, and Excellent is the grade to look for. For fancy shapes such as oval, emerald or cushion, look at the proportions and how evenly the stone reflects light.

Before buying, check the GIA report number online and make sure it matches the number laser-inscribed on the girdle. Above 1.5 carats we choose stones graded F to G in colour and VS or better in clarity, with an Excellent cut on round stones, and every diamond ring we offer has a GIA certificate.