Large D_color, Flawless or Internally Flawless round brilliant-cut diamonds are indeed very rare as diamond cutters normally tend to pick a shape/cut to suit the shape of the rough diamond, in order to minimize losses. In other words they try to maximize for both quality and quantity, rather than maximizing for quality at the expense of quantity. Hence, shapes/cuts other than round brilliant-cut are common among large diamonds, such as oval brilliant-cut, pear-shaped brilliant cuts, heart-shaped brilliant-cuts, cushion-cut, emerald-cut, asscher-cut, radiant-cut etc. As an example we can take the cutting of the 102.79-carat round brilliant-cut Graff Constellation diamond from the 478-carat "Light of Letseng" rough diamond. After a thorough study of the rough diamond by a team of engineers, gemologists and master cutters and polishers of DIAMCAD using the most advanced scanning technology, it was finally decided to cut the rough diamond into six, one main round brilliant-cut diamond and five satellite diamonds - one smaller round brilliant-cut, a heart-shaped diamond, two pear-cut diamonds and a marquise-cut diamond. The largest diamond created was the 102.79-carat, round brilliant-cut Graff Constellation diamond.The second largest was the 51.20-carat heart-shaped diamond. Four other tiny diamonds were also created from the corners of the rough stone, making a total of 9 satellite diamonds. Hence, in creating the round brilliant-cut Graff Constellation, 9 other satellite diamonds were also created at the same time.
In the case of the Light of Letseng rough diamond, the creation of 9 satellite diamonds was possible because of the high quality of the rough diamond. Thus wastage of diamond in cutting was a minimum. If the rough diamond was not of very good gem quality the creation of so many satellite diamonds would not be possible, and there would be enormous wastage in cutting the main diamond.
You have rightly mentioned the three main famous round brilliant-cut diamonds in existence, viz. the 84.37-carat Chloe Diamond, 90.97-carat Icon/Safiya Diamond and the 102.79-carat Graff Constellation Diamond. There are not many D-color, IF/FL, round brilliant-cut diamonds above 50 carats in existence. A round brilliant-cut diamond weighing 103.46 carats cut and polished by Graff and mounted on a platinum ring appeared at a Sotheby's Geneva sale held on May 13, 2014. But the diamond is N-color and SI-1 clarity and sold for US$4.8 million. However, there are quite a number of D-color, IF/FL round brilliant-cut diamonds between 10 to 50 carats that appeared at public auctions and performed significantly well.
It will be interesting to have the images of the 1st, 2nd and 3rd largest D-color, Internally- Flawless, round brilliant-cut diamonds -the Graff Constellation, the Safiya and the Chloe diamonds side by side for comparison.
Today even with the most advanced modern techniques at the disposal of diamond cutters, the cutting and polishing of a rough diamond crystal, always results in a dramatic loss of weight, which is rarely less than 50%. The round brilliant-cut is generally preferred when the rough diamond crystal is an octahedron, as it is often possible to cut two round brilliant-cut diamonds from one such crystal, cutting the rough diamond crystal into two right across the center of the crystal and aligning the girdle and culet of the expected finished diamonds towards the pointed ends of the crystal. The girdles of the expected diamonds fits into the natural shape of the crystal, reducing weight loss. On the other hand if the rough diamond was cubical in shape, in fashioning the cone-shaped girdle, considerable loss of weight can occur, unless triangular portions are sawed out from both sides of the crystal, and if such portions are of gem-quality smaller diamonds created out of them.
The images uploaded below may serve to give a better understanding of what I have stated :-
1) Image of the geometrical octahedral shape.
2) Image of a natural octahedral diamond crystal.
3) Image of how two round brilliant-cut diamonds can be cut from a single octahedral crystal.
4) Image of a cubical shaped rough diamond crystal
Thank you "themonetaryman" for revealing the actual aim of your original question seeking information on D-color RBC diamonds of clarity grade IF/FL above 50 carats in weight. While wishing you Good Luck in your quest for the rare diamond, I feel that we should thank you profusely for your contribution, as it has generated a discussion on various aspects of large D-color round brilliant-cut diamonds, such as there scarcity etc. I also feel that this thread on RBC diamonds would help to bring into focus all such diamonds that have performed extremely well at public auctions, including those that are less then 50 carats in weight. I invite all active contributors to our forums to help bring such RBCs to this forum and highlight their existence. Thank you !
@themonetaryman Thank you very much.
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Let me introduce you to two world record holders among D-color, RBC diamonds of Flawless/Internally Flawless clarity grade, less than 50 carats in weight.
One D-color RBC diamond of Flawless clarity weighing 28.86 carats sold for USD6,940,822 at Sotheby's Hong Kong Magnificent Jewels & Jadeite Sale on April 8, 2013 (Lot 1659). This works out to a price-per-carat value of USD240,500, a world record for price-per-carat for a round brilliant-cut D-color diamond.
The other D-color, RBC diamond of Internally Flawless clarity weighing 25.32 carats sold for USD 6,246,702 at Sotheby's Geneva Magnificent Jewels Sale held on May 13, 2014 (Lot 487). This works out to a price-per-carat value of USD 246,710, a new world auction record for price-per-carat for a round brilliant-cut, D-color diamond.
Thank you for that, now to find one 50-100% larger 
Another D-color, Flawless Clarity, round brilliant-cut diamond weighing 30.57 carats (Lot 1910), with excellent cut, polish and symmetry made a very significant impact at the Sotheby's Hong Kong Magnificent Jewels & Jadeite Sale, held on April 7, 2014, by registering a price of USD6,585,744 within the pre-sale estimate of USD5,930,780 - 6,833,290. This works out to a price-per-carat value of USD215,431, much below the world record price-per-carat value of USD240,500 set an year earlier on April 8, 2013, and subsequently renewed on May 13, 2014 at Sotheby's Geneva by registering a ppc-value of USD246,710.
For comparison sake, It will be interesting to find out the price-per-carat values of the three largest D-color, Flawless/Internally Flawless, round brilliant-cut diamonds in the world, the Chloe diamond, the Safiya diamond and the Graff Constellation diamond.
Of the three RBC diamonds you have mentioned Joan, The Graff Constellation diamond is yet to be sold, the Safiya diamond was sold by Graff in September 2000, to an unidentified Middle-Eastern buyer, however, the identity of the purchaser and the price of purchase was not revealed. Hence, we are left only with the Chloe diamond, which was sold at a Sotheby's Magnificent Jewels auction held in Geneva, on November 14, 2007, nearly eight years ago, for US$ 16,189,769 million (18.2 million Swiss francs). At that time the 84.37-carat Chloe diamond set several auction records, such as the largest, D-Color, flawless, round brilliant-cut diamond ever sold at an auction; the second most expensive jewel ever auctioned, the first being the 100.1-carat, D-color, pear-shaped, internally flawless, "Star of the Season" diamond which was sold in 1995, also by Sotheby's for US$16.5 million, to the Saudi-based renowned jeweler and diamond collector, Sheik Ahmed Hassan Fitaihi; the highest price-per-carat ever paid for a D-color, flawless/internally flawless diamond at an auction. The price-per-carat of the chloe diamond works out to US$191,890. In comparison the price-per-carat of the "Star of the Season" diamond was only US$ 164,835.
It is important to note that the whole stone price as well the price-per-carat of the 84.37-carat Chloe diamond was in accordance with the market values applicable in the year 2007 and hence may not be used for comparison with the price-per-carat value of diamonds sold in the year 2015. Like colored diamonds, whole stone prices as well as price-per-carat values of D-color, flawless/internally flawless diamonds, also have significantly increased particularly after the Great Recession of 2008/2009, when investors sought for safe havens to invest their money. The world record price-per-carat values of US$240,500 and US$246,710 realized respectively by the 28.86-carat and 25.32-carat D-color, flawless/internally flawless RBC diamonds is a reflection of the significant increase that took place after year 2009. Hence, the actual PPC-values of the Chloe-diamond, Safiya diamond, the Graff Constellation diamond or any other comparable RBC diamond will be known only when such a diamond appears at a public auction.
Thanks Lareef for for your detailed reply in response to my post. However, won't the staggering prices achieved recently by some D-color diamonds of Flawless/Internally Flawless clarity greater than 100 carats in weight but with shapes/cut other than round brilliant-cut, give an indication of the ppc-values of D-color RBCs greater than 100 carats in weight ?
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