The pear-shaped colourless diamond (below) is estimated to sell for $20m (€15.46m) at an auction in Switzerland in a few weeks.
The D-colour, flawless stone, weighing 101.73 carats, will feature in a jewellery auction by Christie's International at the Four Seasons Hotel des Bergues, Geneva, on May 15.
It is the largest colourless diamond of this optimum grade to appear at auction and will be sold by a client who wishes to remain anonymous. This is the first time the gem has appeared for sale and the buyer will be able to name it, the London-based company said yesterday.
The stone was recently cut from a rough diamond, weighing 236 carats, that had been found at the Jwaneng mine in Botswana. It took 21 months to polish.
Demand for the rarest diamonds as an alternative asset and portable store of value has pushed up prices in recent years.
The Jwaneng (meaning "place of small stones") open-pit diamond mine, the world's richest by value, is jointly owned by De Beers and the government of Botswana.
At the top end of the market, "white" stones are regarded as less desirable than their even rarer coloured equivalents.
The record price for any gem at auction is the 45.4 million francs paid by the London dealer Laurence Graff for a 24.78-carat emerald-cut "Fancy Intense Pink" at Sotheby's, Geneva, in November 2010.
The record auction price for a colourless stone is the 20.4 million Swiss francs with fees paid for the 76.02-carat Archduke Joseph Diamond at Christie's, Geneva, in November last year.