"The 20.46 carat Okavango Blue of Botswana is one of the rarest and most remarkable blue diamonds ever discovered," the Okavango Diamond Company said.

The diamond was discovered last year as a 41.11 carat rough stone at the Orapa mine operated by Debswana, a joint venture between Botswana and De Beers.

"From the first moment we saw the diamond, it was clear we had something very special," Okavango MD Marcus ter Haar said.

"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."

The Gemological Institute of America has graded the Type IIb Fancy Deep Blue as an oval brilliant cut, VVS2 clarity, making it one of the highest polished colour classifications attainable for any blue diamond, Okavango said.

It was named Okavango Blue after Botswana's World Heritage-listed Okavango Delta.

The Hope Diamond's cut was "not as precise or beautiful as ours and the purity is lower even though it's bigger" at 45.5 carats, ter Haar told Bloomberg.

He did not speculate on a price the Okavango Blue might fetch.