The Letseng mine in Lesotho, has yielded a 254 carat Type II white diamond.
The deposit, owned by London-based Gem Diamonds, is the highest dollar per carat kimberlite diamond mine in the world, and last year achieved an average value of $1,908 per carat.
Its highest per carat value for a white rough diamond during the year was $38,827.
The new stone (pictured) is the first major find announced since a 126.9-carat type II diamond in January.
Gem Diamonds saw an increase in revenue and profits in 2020, despite Covid forcing the closure of Letseng between March and May.
During the year it recovered 16 diamonds larger than 100 carats, compared with 11 in 2019.
Letseng’s most famous stone is the 910-carat Lesotho Legend, which sold for $40m in 2018.