The Letseng mine - properly Letseng-La-Terae - is an open-pit kimberlite diamond mine in the Maluti Mountains of north-eastern Lesotho, at an altitude of approximately 3,100 metres above sea level, the highest active mining operation in Africa. It is operated by Letseng Diamonds Ltd., a joint venture between the Australian-listed Gem Diamonds Ltd. (70%) and the Government of Lesotho (30%), and is the world's most valuable kimberlite mine by carat - in-situ revenue per carat exceeds U.S. $1,500 and frequently several thousand, well above any other large kimberlite operation - despite producing only modest tonnage of rough.

Geology and grade

Letseng comprises two adjacent kimberlite pipes, the Main Pipe and the Satellite Pipe, intruded into Karoo basalts of the Maluti Mountain range. The grade is very low by industry standards - approximately 1.5 to 2 carats per hundred tonnes (cpht), among the lowest of any commercial kimberlite mine - but the size and quality distribution is extraordinary. A high proportion of the production comprises plus-100-carat Type IIa stones, the chemically pure, nitrogen-free goods that yield D-colour, internally-flawless polished diamonds. The combination of ultra-low grade with ultra-high stone quality is the defining characteristic of Letseng and is unique in the modern kimberlite industry.

History and ownership

The Letseng-La-Terae area first attracted commercial attention with the 1967 recovery of the 601-carat Lesotho Brown rough (giving rise to the 71.73-carat Lesotho I, the 60.61-carat Lesotho II and the 40.42-carat Lesotho III, the latter the Onassis-Kennedy stone). De Beers operated the area as a small mine from 1977 to 1982 but closed it citing low grades. The operation was revived by South African company Trans Hex in 1998-2000 and then taken over by Gem Diamonds in 2003, which has operated it as Letseng Diamonds since 2006 in joint venture with the Lesotho government.

Notable stones

Letseng's modern era under Gem Diamonds has been defined by a series of exceptional large-stone recoveries. Among the most significant:

• Lesotho Promise - 603 carats, August 2006, sold for U.S. $12.4 million.
• Letseng Legacy - 493 carats, September 2007, sold for $10.4 million.
• Letseng Star - 550 carats, August 2011.
• Lesotho Star - 550 carats, January 2018.
• Anonymous large stones - over twenty plus-100-carat stones recovered between 2006 and the early 2020s, including several plus-200- and plus-300-carat stones.

The Lesotho Promise was cut by Graff Diamonds into 26 polished diamonds, the largest a 75-carat pear-shape D-colour. The Letseng Legacy was cut into a series of polished stones the largest of which was approximately 75 carats.

Trade significance

Letseng is the reference operation for high-value, low-grade kimberlite mining and the dominant source of plus-100-carat Type IIa stones in the contemporary market. The mine's rough is sold through tender at Antwerp and through specialist large-stone auctions at Sotheby's and Christie's. The Government of Lesotho's 30% interest provides substantial royalty and dividend income to the Treasury and makes Letseng one of the country's principal sources of foreign-exchange earnings. Mine life is currently estimated to extend into the 2030s on existing reserves, with potential for further extension through deeper open-pit phases or transition to underground mining at the higher-grade portions of the Main Pipe.