The Williamson Diamond Mine (also known as the Mwadui mine) is a diamond mine northeast of Shinyanga in Tanzania; it became well known as the first significant diamond mine outside of South Africa. The kimberlite pipe was found in March 1940 and the mine established by John Williamson, a Canadian geologist, and has been continuous operation since then, making it one of the oldest continuously operating diamond mines in the world. Over its lifetime it has produced over 19 million carats (3,800 kg) of diamonds. A 2020 report by The Guardian said that high-quality pink diamonds from the mine could value up to $700,000 a carat.
The Williamson mine was once owned by its namesake Williamson and later nationalized by the government of Tanzania. Since February 2009 the mine is mostly owned by Petra Diamonds, with 75% ownership, the government of Tanzania owning the remaining 25%. In 2020, the mine came under scrutiny because of alleged human rights violations.
Williamson was hired by the Mabuki diamond mine, in 1938 and used it as his base for diamond prospecting in the region. In 1940 he discovered the Mwadui kimberlite pipe; The mine was noted for technical innovations in diamond mining which were developed under Williamson's watch. Williamson closely managed the mine until his death in 1958 at the age of 50.
Williamson's heirs sold the mine NGO RAID claimed evidence of harm by contracted guards who shot locals on the concession, including 10 killings and 50 injuries since 2009. Initial internal investigations found record of only one live-fire incident during the alleged window, alongside other use of "reasonable force" against illegal miners.