An extraordinarily rare blue diamond ring is expected to fetch as much as £2 million when it goes up for sale next month.

Auctioneers Elmwood’s stated that the jewel will be only the third blue diamond weighing over eight carats ever offered at auction in the UK.

Featuring an 8.03 carat Fancy Light Blue diamond, the piece is set to appear in Elmwood’s Fine Jewellery sale in London on 1 October.

Elmwood’s said the only other blue diamonds weighing more than eight carats ever auctioned in the UK are the 70.21 carat "Idol’s Eye", sold at Christie’s in London in 1865, and the Wittelsbach Diamond, sold at Sotheby’s in London in 2008.

The pear-shaped diamond ring is being sold by a collector in Glasgow and carries an estimate of £1.5 million to £2 million.

Joe Kendrick, head of sale at Elmwood’s auctioneers, said: “An 8.03 carat natural blue diamond is an extraordinary rarity in its own right, but what adds to its allure is its place in British auction history.

“To offer only the third blue diamond above eight carats ever seen at auction in the UK and the largest pear-shaped example, is a remarkable moment for Elmwood’s and for collectors of exceptional coloured diamonds.

“Coming from a Scottish collector, the ring is a truly once in a lifetime piece.

“Its combination of size, colour, clarity and elegant pear shape also gives this stone an exceptionally compelling presence and we therefore expect wide interest.”

The blue diamond is set at the centre of the ring, which also has concentric borders of round brilliant cut pink and white diamonds.

Natural blue diamonds are described as being among the rarest of all coloured diamonds.

Their colour is linked to trace amounts of boron incorporated into the gem’s crystal structure.

In the past, blue diamonds were mainly sourced from India and Brazil, however in more recent times they have also been found in South Africa’s Cullinan Mine.