The $28m diamond as big as a pound coin

A pink diamond the size of a pound coin is being put up for auction by Christie's International for an estimated $28m (£18.3m).

The stone, which was bought 20 years ago for $10m, is expected to break sales records when bidding begins in Geneva on 10 November.

It is one of the rarest colours and sizes to ever come to the market.

Christie's says only four other similarly shaped and sized diamonds have been sold at auction in 250 years.

The 16.08 carat gem, specifically a cushion cut fancy vivid pink diamond, is the largest of its kind to be auctioned.

"Fancy vivid" refers to the intense shade of pink and is the highest colour ranking a diamond can be given.

Investment

"This is the Rolls Royce of diamonds," said Rahul Kadakia, Christie's head of jewellery. "Every major royal collection in the world has a pink diamond and this is the one of the largest to come up for auction."

The gem is cut in a cushion shape and set in a ring, which may increase its desirability for some, but is not expected to affect the value.

"There is interest in these not only as treasures to wear and enjoy, but also as investments that will increase in value," said Melvyn Kirtley, chief gemmologist for luxury jewellery brand company Tiffany & Co.

The diamond's seller has seen their investment more than double in value over the time they have held it.

"For investors worried about where we are going to go in the investment world this is a stable form of value," said Mr Kadakia.

Christie's priced the stone between $23m-$28m with the intention of attracting collectors, traders and investors. Mr Kadakia said he expects the price could go higher.

A similar shaped and coloured diamond sold for more than $2m per carat in 2009.

Rising popularity

In 2013, Sotheby's sold the world's largest fancy pink diamond, the 59.6 carat Pink Star, for an estimated $83m.

Throughout history there have been famous pink diamonds, including the Williamson, part of the UK's Crown Jewels, and the Hortensia, formerly part of the French Crown Jewels.

However, the popularity of pink diamonds for engagement rings and other pieces of jewellery has only boomed in the past few decades.

Much of the growing demand has come from the increased availability, which occurred when the Argyle diamond mine in Western Australia opened in 1985.

Pink diamonds though are still rare and an engagement ring at stores like Tiffany can cost more than $6m.

Coloured diamonds are among the rarest in the world, even ones that are not particularly vivid or clear.