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On May 27th, 1971, Christie's auction house in Geneva offered to prospective buyers a cushion-cut, 104.52 carat (20.904 g) fancy golden-yellow diamond which would later come to be known as the Deepdene.

Its clarity was stated as VVS1 (with two very small imperfections near the girdle or midsection of the stone) and its vivid-yellow colour guaranteed to be natural by both the German gemmological institute and the University of Mainz.

However, Dr. Edward Gübelin, also of Geneva, had the opportunity to inspect the diamond before the auction took place. Gübelin had a keen eye and experience with examining gems; he was adept at recognizing an artificially colored diamond. The Deepdene had been irradiated, likely by neutron bombardment. Gübelin warned Christie's and any buyers who would listen, but the sale was allowed to continue with the recommendation that it be examined by another lab.

After being sold to jewellers Van Cleef & Arpels for a reported £190,000, the Deepdene was sent to the Gem Testing Laboratory in London, England. The lab's director, gemmological pioneer Basil Anderson, was given the task of substantiating Gübelin's claims. That he did, using spectral analysis with damning results.

Van Cleef & Arpels promptly returned the unfortunate stone for a refund, leaving it somewhat of an unwanted orphan. Its current whereabouts remain uncertain.

The Deepdene gets its name from the Pennsylvania estate of Mrs. Bok, wife of Cary W. Bok, both the diamond's original owners. At the time, the Deepdene weighed slightly more at 104.88 carats (20.976 g) and was mounted in a diamond clip. Harry Winston bought the diamond from the Boks in 1954 and it eventually found its way to a London firm in 1960 and later to its German owner. Assuming Winston did not have the irradiation treatment done, it could have taken place any time between 1955 (when Winston sold it) to 1971, the year it was offered at auction. In any event, the Deepdene is widely considered to be the largest irradiated diamond in the world.

Deepdene is also the name of a Melbourne, State of Victoria, Australia locality which is a part of the greater suburb Balwyn.

An area of the Surrey town Dorking, England.