World's most valuable pink diamond up for auction

The world's most valuable cut diamond is up for auction - and expected to sell for more than £48 million.

Despite being small enough to fit on a ring, the Pink Star is said to be the the largest internally flawless fancy vivid pink diamond ever to be graded by the Geological Institute of America.

Rare pink diamond expected to fetch millions

It was initially mined by De Beers in 1999 as a 132.5 carat rough diamond. After being cut and polished to an oval shape - a process that took two years - it ended up at 59.6 carats.

That's twice the size of the nearest pink contender, the 24.78-carat Graff Pink, which sold in 2010 for $46.2 million.

The Pink Star diamond
The Pink Star diamond

"The extraordinary size of this 59.60-carat diamond, paired with its richness of colour, surpasses any known pink diamond record in history," said David Bennett, worldwide Chairman of Sotheby's Jewellery Division, tells Reuters.

'Sky Blue' diamond expected to fetch £20m at auction

It's up for sale in Hong Kong on April 4, with Sotheby's predicting that it could go for $60 million - beating the current world record, the Oppenheimer Blue diamond, which sold for $57.5 million at Christie's in Geneva last May.

In fact, though, the Pink Star nearly made an even higher price a few years ago. Back in 2013, it went under the hammer in Geneva and was sold for an eye-watering £52 million, making headlines around the world.

However, the buyer, New York-based diamond cutter Isaac Wolf, defaulted on his payment, leaving the stone in Sotheby's inventory ever since.

Coloured diamonds have been particularly popular in recent years. They get their hues from impurities or structural defects and come in every shade from blue, yellow, red, green and blue to even black.

Lab turns human ashes into sparkling diamonds

Prices have been rising: according to the Fancy Color Research Foundation, the price of pink diamonds rocketed by 180% between 2009 and September last year; in the same period, blue and yellow diamonds rose by 70% and 90% respectively.

It's mostly to do with rarity: coloured diamonds are far scarcer than the white kind, with pink diamonds the rarest of all.

If you have a diamond and you're wondering what it's worth, the 'four Cs' provide a guide: carat (its weight), colour, clarity and cut. But it's not a job for the amateur, so you'll need to go to an expert - and, be warned, if you've bought it in the last ten years, you're highly unlikely to get back what you paid for it.