Tresor Paris: How to choose the perfect diamond

Earlier this month, a diamond ring bought for £10 at a car boot sale was sold at auction for £656,750. Its owner had thought that because of its size, the diamond was a piece of costume jewellery and would wear it to go shopping. It took an expert to alert her to how much it could be worth – much to the woman's surprise.

But how can you tell what makes a good diamond? And what should you look for when you're buying one? The Week asked Lilian Lousky and Salim Hasbani of multiaward-winning jewellers Tresor Paris for their advice.

"When you're looking for a diamond, you have the 'four Cs': carat, cut, colour and clarity," says Salim.

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Carat

Diamonds are measured on how much they weigh, using the metric "carat". Each carat can be broken down into 100 "points" to allow for precise measurement. The higher the carat, the more expensive the diamond. However, the number of carats does not reflect the size of the diamond – two diamonds of equal carat might different sizes depending on the cut.

Cut

The "cut" refers to the shape of the diamond. "The most popular shape is round classic," says Salim, but there are many different options, such as emerald, pear and marquise. A well-cut stone will bring out the light, reflecting it back out, and the diamond's "fire" – the coloured sparkle when light hits the stone and you get a rainbow. The better the cut, the more sparkle. If a diamond is cut too shallow or deep, the light may escape through the side, making the diamond look dull.

Colour

Diamond colours are graded from D to J, says Lilian: "D is the whitest diamond colour you can get – the top one."

D colour stones actually have no colour, making them the rarest and best quality diamond, which is reflected in their prices. The colours intensify as you go down the scale, with the stones having an increasing brown or yellow tint.

"The target is a completely colourless diamond," says Salim. "A G colour will look white to the naked eye, but if you put it next to a D, which is only four colours better, you can see the difference in the shade of it. One will be – not whiter, but more colourless. "

Lilian adds: "It's like black dresses – if you put two black dresses together, they're both black but one can be a slightly different shade. It's only when you see them together that you see the difference."

Clarity

Clarity refers to the impurities – "inclusions" - that form inside the diamonds, although these can be so minute they can only be measured by an expert gemologist using a powerful microscope. The fewer inclusions, the purer the diamond – and the rarer. The diamond grades for these inclusions are:

FL-IF: Flawless and internally flawless, where the stone has external marks that can easily be removed with polishing.

VVS1-VVS2: VVS stands for "very, very slightly included", meaning stones that have tiny flaws that can only be seen by a professional using a very powerful microscope. A VVS2 diamond has inclusions in the top half of the stone; if they're in the bottom, it's a VVS1.

VS1-VS2: "Very slightly included" – the flaws are a little easier to see under a microscope, but still take an expert time to find. Typically, these inclusions are invisible to the naked eye.

SI1-SI2: "Slightly included", with inclusions easier to see under a microscope.

I1-I3: "Included", where the flaws can be seen by the naked eye.

Choose a D Flawless diamond and you're paying top end prices, says Lilian. Tresor Paris's top D Flawless ring currently retails for £250,000, although obviously, prices can fall depending on the market - a Financial Times report last year revealed how a surfeit in supply had hit values.

Lilian adds: "Ideally, you want to go the top two colours and the top two clarities. We use the higher ones because they have the biggest sparkle.

"Of course, your budget is important too. Sometimes people go for a smaller diamond because they want a better quality and when you're looking at smaller diamonds, you can see the colour is different but you can't see the inclusions so easily. Go for the best diamond for your budget – you can add to it later on, change the shape of the setting, change the band, but you're always going to have a good diamond.

"If you're buying just the one piece, make sure it matches a lot of outfits," she continues. "You want it to be something you're comfortable wearing and a look that you're happy to be seen in a lot, especially if you're spending a lot of money. I advise people to go for something they're not going to get fed up with quickly."

As for where to buy your diamond, Lilian and Salim have no doubt about the most important quality in your retailer: trust.

"You have to trust the person," says Lilian. "Our clients are here because of word-of-mouth or they've been coming here for years. Go with someone you've been recommended, visit them and see if you trust them. You have to trust the person."

Tresor Paris, 7 Greville St, Hatton Garden, London EC1N 8PQ. tresorparis.co.uk