GABORONE, BOTSWANA (AFP) - Botswana's state-owned Okavango Diamond Company on Wednesday (April 17) unveiled the biggest blue diamond ever discovered in the southern African country.

At 20.46 carats, the gem is one of the rarest stones in the world, and "sits in the very top bracket of all-time historical blue diamond finds", the company said in a statement.

The oval-shaped diamond has been named "The Okavango Blue" in honour of the Okavango Delta, the country's wildlife-rich world heritage site.

"It is incredibly unusual for a stone of this colour and nature to have come from Botswana - a once-in-a-lifetime find, which is about as rare as a star in the Milky Way," said Okavango Diamond Company managing director Marcus ter Haar.

"It is little surprise blue diamonds are so sought after around the world as only a very small percentage of the world's diamonds are classified as fancy colour," he added.

Botswana is one of the world's largest producers of diamonds, which are its main source of income and account for about 80 per cent of its exports.