An extraordinarily rare white diamond is set to sell at Sotheby's Diamond. The 102.34 carats stone, billed as the world's largest-known round, D-colour, Flawless diamond, is expected to fetch over US$33m.

The diamond was cut from a 425-carat rough stone mined by the De Beers in Botswana, South Africa. It was then handed to artisans in Johannesburg and New York for a cutting and polishing process, which took over six months.

In addition to its enormous size, the rarity of the diamond is also defined by achieving perfection in all four critical criteria. It is D colour, the highest grade for a white diamond; of exceptional clarity (it is completely flawless, both internally and externally).

The stone is part of the rare subgroup comprising less than 2% of all gem diamonds, known as Type IIa. Diamonds in this group are the most chemically pure type of diamond and often have exceptional optical transparency.

It is not common to find a round brilliant-cut stone as the cutter will generally try to keep the maximum of weight from the rough. That is why elongated cushions, emerald-cuts, and pear-shapes are usually more common for large diamonds.

There were only seven D-colour, colourless diamonds weighing more than 100 carats have appeared at auction. They were mostly of pear-shape and rectangular, none of round brilliant-cut. The rare stone will be sold through private purchase at Sotheby's Diamonds. It is expected to fetch well over $33.7m and break the previous auction records for similar pieces.

In 2015, a step-cut diamond weighing 100.2 carats (image above), also D-colour, internally Flawless, Type IIa, was sold for US$22.1m with buyer's premium at Sotheby's New York.