Absolute perfection! A new colourless pear-shaped diamond weighing 101.73 carats, touted as the world's largest 'flawless' gem ever, will go under the hammer here.

The rough diamond of 236 carats was found at the jwaneng mine in Botswana and took 21 months to polish.

The sensational gemstone is not only one of the largest pear-shaped diamonds known to date, it is also one of the world's most perfect diamonds: a D colour, Type IIA Flawless gem, auction house Christie's said.

"Just as a great artist is able to 'see' a masterpiece in a block of marble, a perfect diamond is the result of the skill and vision of a master cutter who is able to uncover beauty from a rough gem," Rahul Kadakia, Head of Jewellery, Christie's Switzerland and Americas said.

"On 15 May, Christie's Geneva will offer for sale this wonderful pear-shaped diamond of 101.73 carats, which has literally been sculpted like a work of art.

"This diamond is in its scale, quality, and rarity one of the most beautiful 'white' diamonds Christie's has ever had the honour of offering for sale," Kadakia said in a statement.

Meeting every criteria of perfection, the diamond on offer at Christie's Geneva May auction of Magnificent Jewels has been graded by the Gemological Institute of America with the best colour, D, and the best clarity, Flawless.

Its absolute symmetry is a tour-de-force, demonstrating the exceptional skills needed to achieve such a perfect gem.

It also benefits from rare brilliance and inner limpidity that is only to be found in the chemically pure Type IIA diamonds, which account for less than 2 per cent of the world's diamond production, said Christie's.

The world's most famous pear-shaped diamond is the Cullinan I, a colourless, Type IIA stone of 530.20 carats, which forms part of the British Crown Jewels. This is also the largest colourless diamond known, it said.

The current auction record for a colourless diamond was set by the historical 76.02 carats cushion-shaped Archduke Joseph Diamond when it sold for USD 21.5 million at Christie's Geneva in November 2012.

In May 2011, a perfectly symmetrical 56 carat heart-shaped diamond, sold for more than USD 10.9 million becoming the most valuable heart-shaped diamond ever sold at auction.