An extremely rare 12-carat blue diamond, exhibited by Sotheby's in Geneva on Wednesday (4 November), is expected to break a record at auction in a week's time with a price estimate of $35-$55m (£23-36m).The gem, named Blue Moon, was discovered in January 2014 as part of a 29.62 carat blue rough diamond at Cullinan mine in South Africa, the world's main source for blue diamonds. Sotheby's International Jewellery Division Chairman and Auctioneer, David Bennett, said that experts had deemed it internally flawless due to its clarity, colour and size.

"The GIA (Gemmological Institute of America) graded, I think, 400 blue diamonds and only 4 of the 400 were graded vivid blue, so even among this extremely rare class of blue diamonds, to get one vivid blue, 12 carats, internally flawless, is once in a blue moon," he said.

The grade of "fancy vivid blue" awarded by the GIA is the highest possible colour grading for a diamond of its kind. Bennett said that American experts had deemed its colour as "indescribable".

"This colour has been described by the Gemmological Institute of America as being almost indescribable. I think the closest that anybody has said is like looking at the play of the sun on the sea against this background, this wonderful sort of blue you want to dive into, you know. I find it endlessly fascinating, every time I look at it it's different," he said.

So far, the highest price on record for a blue diamond is $32.6m, paid in November 2014 in New York for Bunny Mellon's Blue Diamond, but this is expected to be broken by a considerable amount next week. According to gemmologists, a blue diamond receives its colour from small amounts of the chemical element boron which becomes trapped in the crystal structure during its formation.

"You're looking at something that, by freak of nature, has been brought up by a volcano. It is just so extraordinary that this diamond would have stayed forever, miles under the earth surface but by the fact that it was associated with a volcano is now part of our world, I find that absolutely extraordinary. It's like the most secret part of the earth is just being shown to us," Bennett said.

The gem is the star lot of Sotheby's Magnificent Jewels and Nobel Jewels auction and has been exhibited in Hong Kong, London and New York before arriving in Geneva to go up for auction on 11th November.