A Diamond Necklace. Photo Christie's image Ltd 2012

Suspending a rectangular-cut diamond, weighing approximately 6.01 carats, within a circular-cut diamond surround, to the graduated circular-cut diamond neckchain, mounted in platinum, 17 ins. Estimate $125,000 - $200,000

With report 12595359 dated 4 September 2012 from the Gemological Institute of America stating that the diamond is H color, SI2 clarity 

Christie's. Magnificent Jewels. 16 October 2012. New York, Rockefeller Plaza