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.