The stone is a D colour Type II diamond of exceptional quality, Gem Diamonds said in a statement on Friday.
The shares jumped as much as 11percent, the most since January, and traded at 98.25 pence by 8.45am.
The company's Letseng mine is renowned for the size and quality of its stones with its diamonds selling at an average price of almost $2 000 (R27 500) a carat, the highest in the industry.
In comparison, the global average is about $120 a carat.
Still, the company has suffered recently from a lack of big finds, with the miner discovering just five stones bigger than 100 carats last year.
Still, the diamond is not even close to the biggest found at Letseng.
In 2015 it sold a 357-carat stone for $19.3 million and in 2006 found the 603-carat Lesotho Promise.
The mine has produced four of the 20 largest gem-quality white diamonds recorded.