Miners in Angola have unearthed a rare pure pink diamond that is believed to be the largest found in 300 years, the Australian site operator has announced.

A 170 carat pink diamond — dubbed The Lulo Rose — was discovered at the Lulo alluvial diamond mine in the country's diamond-rich north-east, and is among the largest pink diamonds ever found, the Lucapa Diamond Company said in a statement to investors on Wednesday.

The Lulo mine has already produced the two largest diamonds ever found in Angola, including a 404-carat clear diamond.

The pink gemstone is the fifth-largest diamond found at the mine where 27 diamonds of 100 carats or more have been found, according to Lucapa.

Angola's mines make it one of the world's top 10 producers of diamonds.

The "historic" find of the Type IIa diamond, one of the rarest and purest forms of natural stones, was welcomed by the Angolan government, which is also a partner in the mine.

"This record and spectacular pink diamond recovered from Lulo continues to showcase Angola as an important player on the world stage," Angola's Mineral Resources Minister Diamantino Azevedo said.

The pink diamond will be sold by international tender by the Angolan state diamond marketing company, Sodiam, likely at a dazzling price.

Although The Lulo Rose would have to be cut and polished to realise its true value, in a process that can see a stone lose 50 per cent of its weight, similar pink diamonds have sold for record-breaking prices.

The 59.6 carat Pink Star was sold at a Hong Kong auction in 2017 for US$71.2 million (more than $102 million). It remains the most expensive diamond ever sold.

The pink diamond is an impressive size but many clear diamonds are larger than 1,000 carats.

The Cullinan diamond found in South Africa in 1905 tips the scales at 3,106 carats and is in the British Sovereign's Sceptre.

AFP / AP