One of the largest diamonds ever found has been unearthed at a mine in Lesotho, southern Africa.

The 910-carat colourless diamond was discovered at the Letseng mine owned by UK-listed firm Gem Diamonds, who claim the stone is the fifth largest in history.

Analysts forecast the gem could be worth as much as $40m, but its value will ultimately be determined by the by the size and quality of the polished stones that can be cut from it. It will take jewellers around two years to cut and polish the stone, which is the size of two golf balls.

"The pricing of diamonds is hugely variable and driven by a multitude of factors," said Liberum Capital Markets analyst Ben Davis. "But assuming that there are no large inclusions [flaws] running through the diamond, we initially estimate a sale of $40m."

The broker based this price on the 1,109-carat Lesedi la Rona diamond found in the Karowe mine in Botswana in 2015, which eventually fetched $53m.

The news comes as a welcome boost to Gem Diamonds, which bought the Letseng mine in 2006. The site has a history of seismic diamond discoveries, but has not made a significant find for almost three years, depressing the firm's market value over that period.

Shares in Gem Diamonds jumped 18% to 95p, valuing the company at £128m in morning trading (15 January).

"This exceptional top-quality diamond is the largest to be mined to date and highlights the unsurpassed quality of the Letseng mine," said the firm's chief executive Clifford Elphick.

The largest stone ever discovered is the 3,106-carat Cullinan diamond, found near Pretoria, South Africa, in 1905. The gem was cut into the Great Star of Africa and the Lesser Star of Africa, which both form part of the British Crown Jewels.