The Williamson mine is an open pit diamond mine located at Mwadui in the Shinyanga Province of northern Tanzania. The mine is operated by Petra Diamonds.
At 146 hectares, Williamson is the largest kimberlite pipe ever to be mined economically, having been operated continuously as an open pit mine for almost 70 years (since 1940). During this time it has produced over 20 million carats, and there remains a major resource of some 40 million carats. The mine regularly produces large, high-quality stones and is a source of rare and extremely valuable fancy pink diamonds.
In February 2009, Petra announced that it had completed the acquisition of a 75% interest in the Williamson mine from De Beers. The rest 25% is owned by the Government of the United Republic of Tanzania: 25%. 
Total reserves of Williamson mine are estimated at 40.2 million carats 
The Williamson diamond mine is now characterized by low ore grade of about 6 carats (1,200 mg) per hundred tons (12 mg/t) of ore. This is a dramatic decrease from its production in the mine's early life; during the first 25 years of operation, average ore grade was about 30 carats (6.0 g) per hundred tons (60 mg/t); in the first few years of full operation grades were as high as 62 carats (12 g) per hundred tons (124 mg/t), or 10 times the ore grade realized today. Production in the 1950s and 1960s was usually between 500,000 and 750,000 carats (100 and 150 kg) per year; the peak year of production was 1966, when 924,984 carats (185 kg) were produced. Today production levels for all Williamson mine diamond recovery activities are about 300,000 carats (60 kg) per year.
Notable stones produced at the Williamson mine include 54 carat (10.8 g) flawless pink diamond (presented to then-Princess Elizabeth and Prince Philip upon their wedding in 1947), and a 388 carat (77.6 g) diamond found in 1990.