blue star sapphire
A blue star sapphire is named so because of star-like reflection produced on its surface from the light trapped inside Ligar/Wikimedia Commons

Gem experts in Sri Lanka claim to have found world's largest blue star sapphire stone. Valued at about $100m (£68m), the gemstone was mined in the southern Sri Lankan city of Ratnapura, known as the city of gems.

The blue star sapphire, identified by the distinctive star-like mark found at its centre, weighs 1404.49 carats. According to the gemology institute of Colombo, this is the largest ever weight recorded for certified blue sapphire, BBC reported. The previously known largest blue star sapphire weighed 1,395 carats, it said.

The current owner of the gem said that the star sapphire is estimated to fetch $175m (£119m), if auctioned. "The moment I saw it, I decided to buy," the anonymous owner was quoted as saying in BBC World Service's Newsday programme.

"When the stone was brought to me I suspected that it might be the world's largest blue star sapphire. So I took a risk and bought it." He said that he bought it thinking that it "was not a piece of jewellery but an exhibition piece".

The owner has named the stone The Star of Adam after an Islamic legend which has it that Adam was sent to Sri Lanka from the Garden of Eden. A leading gem and jewellery dealer in Sri Lanka, Armil Samoon, has confirmed to BBC that this is actually the largest blue star sapphire in the world.

Mineral scientist Simon Redfern from the University of Cambridge told BBC Earth the stone could have formed within the granite rocks of Sri Lanka's highlands. He said that it was able to grow so big because of constant but slow change in temperatures and pressures within the rocks for millions of years.

Over so many years, Sri Lanka has been one of the main producers of blue sapphires in the world. While small stones are quite prevalent in the country, large sapphires like the one found in Ratnapura are extremely rare.

In 2011, Sri Lanka's Gem and Jewellery Association said that the blue sapphire in the engagement ring of Kate Middleton that originally belonged to Princess Diana was mined in the country in the 1970s.

Kate middleton's sapphire and diamond engagement ring
A pair of photographs show Kate Middleton (L) in November 2010 and Princess of Wales in August 1996 wearing the same blue sapphire and diamond engagement ring. The sapphire used in the ring is said to be from Sri Lanka REUTERS/Paul Hackett/Dan Chung
More on Sri Lanka