A big blue diamond weighing more than 20 carats has been unveiled in Botswana, where the gem was discovered.

The government-owned Okavango Diamond Company on Wednesday called it the largest blue diamond ever discovered in the southern African nation.

A company statement says it is "in the very top bracket of all-time historical blue diamond finds."

The bright blue colour is attributed to the mineral boron, which was present in the rocks of oceans when the diamond was formed some 1 to 3 billion years ago.

Company managing director Marcus ter Haar says just a few such blue stones have come to market in the last decade.

The company says the diamond is expected to be sold near the end of the year.

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