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(Gemologists in Colombo say the gem is worth at least $100 million, but could sell for up to $175 million.
Star sapphires get their name from the six-pointed star-shaped reflection that appears on their surface when they are polished.
According to local media, the gem was discovered last August in Ratnapura, a town in southern Sri Lanka known as the "City of Gems."
The current owner, who refused to be identified, told Ceylon Today the sapphire was "destined to be a priceless museum piece" and that the stone is so big it "cannot set in any kind of jewelery."
Kate Middleton's engagement ring increased worldwide sales and demand for sapphires.
The new owner would not reveal how much he paid for the gem.
"The moment I saw it, I decided to buy. I suspected that it might be the world's largest blue star sapphire. So I took a risk and bought it," the owner told the BBC.
He is said to have called the gem "The Star of Adam," in line with the Muslim belief that theProphet Adam came to Sri Lanka when God banished him from the Garden of Eden.
Sri Lanka's Minister of Industry and Commerce Rishad Bathiudeen said the country's gem exports had increased by 135%, from $70 million in 2010 to $165 million in 2014.
"It is projected that the global gem and jewelery demand would increase at an annual rate of 6% till 2020," Bathiudeen said in 2015.
The surge in demand may have been sparked by interest in the Duchess of Cambridge's engagement ring, formerly owned by Princess Diana, which features a large Ceylon Blue sapphire.
The National Gem and Jewellery Authority of Sri Lanka said the royal ring had increased sales and demand for Sri Lankan sapphires around the world.
