One of the world's largest-ever diamonds has been unearthed in Botswana.

The 2,492-carat gemstone mined by Canadian company Lucara Diamond Corp this week is believed to be the largest found in a century and the second-largest ever mined. 

Botswana's President Mokgweetsi Masisi showed off the stone in his office, with the government confirming it was the biggest diamond found in the country.

Lucara chief executive William Lamb said the company was "ecstatic" about the latest recovery from its Karowe Diamond Mine, which had produced diamonds of note since its inception in 2012.

"This find not only showcases the remarkable potential of our Karowe Mine, but also upholds our strategic investment in cutting-edge XRT technology," he said in a statement.

"This discovery reinforces Karowe's position as a truly world-class diamond mine and highlights the continued success of our operational and underground development strategy."

The company credited X-ray technology installed in 2017 for identifying, retrieving and preserving mega diamonds deemed to be of high value.

"It is overwhelming," Mr Masisi said as he presented the diamond at an official ceremony at his office on Thursday, according to AP.

The value of the diamond — determined by a combination of its carats, cut, colour, and clarity — is yet to be determined.

Botswana is the second-largest diamond mining country in the world, trailing Russia, and accounts for 20 per cent of global production.

The industry forms the foundation of Botswana's economy, having constituted more than 60 per cent of total exports over the past two decades.

Karowe Mine alone has produced four other diamonds of more than 1,000 carats in the past 10 years.

In 2015, the 1,109-carat Lesedi La Rona rough diamond was recovered from the site, which at the time was said to be the most valuable in the world, ultimately selling for $US53 million (about $80 million).

Four years later came the 1,758-carat Sewelô, ranking as the world's second largest until this week. It was sold to Louis Vuitton for an undisclosed amount.

The Cullinan remains the largest gem-quality rough diamond ever found, weighing a whopping 3,106 carats. It was mined in South Africa in 1905 and gifted to the British monarchy in 1907.

The diamond was cut into nine large stones and close to 100 smaller stones, some of which became a part of the Crown's jewel collection.