Botswana’s state-owned Okavango Diamond Company on Wednesday unveiled the biggest blue diamond ever discovered in the Southern Africa 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.

“It is incredibly unusual for a stone of this colour and nature to have come from Botswana — a once-in-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 percent of its exports.