Gem Diamonds Ltd. unearned the 117 carat and 110 carat D colour type IIa diamond at the Letting mine, the firm said in a statement.
Type IIa diamonds contain very little or no nitrogen atoms and are the most expensive stones.
Many famous large diamonds, such as the Cullinan, Koh-i-Noor, and Lesedi Lar Rona, are all Type IIa.
The diamond mining company’s stock rose as much as 8.2 percent in London trading.
The news will be seen as extra positive for the company, following Gem’s discovery of at least seven stones bigger than 100 carats in 2017 and five in 2016.
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Gem recovered a dozen diamonds bigger than 100 carats in 2015.
But recent finds do not compare with the largest at Letseng, renowned for the quality and size of its stones, as Gem sold a 357-carat stone for $19.3million in 2015 and in 2006 found the 603-carat Lesotho Promise.
The London-based company owns the Letseng mine in Lesotho and the Ghaghoo mine in Botswana.
Analysts Des Kilalea and Tim Huff of investment bank Canaccord Genuity told Diamonds.net: “While we do not know the final value of the stones, it is likely that good-quality, type-IIa diamonds could fetch more than $25,000 per carat.
“The gradual improvement in Letšeng’s output of large white stones should, we think, see the share price stronger today. It also seems to indicate that the trend back toward large recoveries which emerged in [2017] remains on track.”