The company has discovered the gem-quality rough diamond at the Diavik Diamond Mine in the remote Northwest Territories of Canada, 220km south of the Arctic Circle.
According to the company, the diamond is one of the largest diamonds ever discovered in Canada.
The firm has exclusively exhibited the diamond at Kensington Palace in London.
Rio Tinto will continue to showcase the diamond in London, before returning to Antwerp for careful assessment, as part of the next stage.
Rio Tinto Diamonds managing director Jean-Marc Lieberherr said "We are delighted to showcase this exceptional, two billion-year-old Canadian diamond.
"Its ancient beginnings, together with the fortitude, finesse and innovative technology required to unearth a diamond in the challenging sub-arctic environment, make it a true miracle of nature."
Diavik Diamond Mines president and COO Marc Cameron said "In a landscape so pristine and precious to traditional lifestyles, we have seen and continue to see an inspired collaboration between local indigenous people and a modern mining company."
Rio Tinto owns a 60% stake in Diavik Diamond Mine, and is operator of the mine.
Started production in 2003, Diavik has an annual production of around seven millions carats of large and white gem-quality diamonds.
In June this year, Rio Tinto divested its 78% stake in Murowa Diamonds to RZ Murowa. The company also sold its 50% interest in the Sengwa colliery to RZ Murowa.
