Gem Diamonds (LON:GEMD) shares gained just over 3% after it unearthed a significant find, a 100 carat (ct) white diamond, at its Letseng mine in Lesotho.

The rough diamond is the third 100+ carat stone to be recovered from Letseng in the past two months - a 164ct diamond was sold for US$9mln, and a 103ct yellow diamond was also sold.

Gem also revealed that June’s export sales totalled US$22mln, with an average price of US$2,087 per ct. This reflects an improvement in diamond prices; Gem averaged US$1,599 per ct in the first three tenders of the year. According to City broker Investec this is good news and Gem’s earnings are likely to improve.

Meanwhile, fellow broker SP Angel said: “The recovery of a third large high value stone from Letseng will underscore the reason to buy Gemfields. Letseng was always expected to have high value stones but this expectation was overcast last year by operational problems.

"With operations going better and the development of Ghaghoo, particularly accessing the decline through sand being executed successfully, the news flow should continue on a more positive trend.”

Letseng has a history of producing famous large diamonds. As far back as 1967 the 601 ct ‘Lesotho Brown’ was discovered in the area of the modern day mine.

More recently, in 2006, Gem recovered the 603 ct ‘Lesotho Promise’, the world’s 12th largest white diamond, which was sold for US$12.4mln – and was subsequently cut and polished into 26 flawless stone which all feature in a single necklace worth an estimated US$50mln.

The 478 ct ‘Light of Letseng’, found in 2008, was sold for US$18.4mln and the 550 ct ‘Letseng Star’, the world’s 15th largest white diamond, was found at the mine in 2011.