Gem Diamonds Ltd. rose the most in more than nine months in London trading after discovering a 100-carat white diamond at its Letseng mine in Lesotho, Bloomberg reported.

Gem rose as much as 7.7 percent, the steepest intraday gain since Sept. 6, and traded at 121.5 pence at 8:13 am. The diamond is the third stone larger than 100 carats found at Letseng since May, Gem said in a statement today.

“Following recent sales, we believe that this stone could fetch offers greater than $5 million,” Numis Securities Ltd. said in a note to investors.

Letseng, once operated by De Beers, was bought by Gem in 2006 for $118.5 million. The mine has produced some of the world’s biggest diamonds including the 603-carat Lesotho Promise, the largest diamond unearthed this century, and a 550-carat stone that was discovered in 2011.

Gem said that its average selling price was $2,087 a carat in June compared with $1,599 a carat in its first three tenders of 2013.