Pink Legacy, weighing in at just under 19 carats, was bought by US brand Harry Winston at auction in Geneva.
Once owned by the Oppenheimer family, who formerly ran the De Beers mining company, the diamond was referred to as "one of the world's greatest diamonds" by Christie's international head of jewellery, Rahul Kadakia.
"You may see this colour in a pink diamond of less than one carat," Mr Kadakia said. "But this is almost 19 carats and it's as pink as can be. It's unbelievable."
According to www.bbc.com