Details of the visit were not disclosed.

In the 1940s De Beers was indicted by the US Justice Department for price fixing under the Sherman Act, meaning not a single member of the Oppenheimer family may set foot on US soil, not even for a visit. The US also believed De Beers rationed the supply of tool diamonds to the US in World War II, severely damaging their war effort. Legislation was passed to set up a national diamond stockpile. De Beers needed a way to ensure it was the source of this stockpile despite being indicted, Roberts alleges. It sought a middleman to do the deals with the US. Early in the 1950s, Tempelsman met with the Oppenheimers who rule De Beers and became this middleman.

He still buys diamonds from them today, including from Namibia. Allegedly operating with the CIA, he is said to have arranged diamond deals with Zairean president Mobutu and Savimbi of UNITA.

"Tempelsman served the De Beers diamond cartel by promoting US foreign policy decisions that favoured its access to and control of African diamond fields. This lead to the US covertly supporting undemocratic and corrupt regimes in Africa to the great detriment of the African people", according to Janine Roberts.

In 2002,Tempelsman offered Namibia an N$ 800 million loan and in return wanted the government to sell all diamonds produced to De Beers only. This offer was declined after the letter was leaked to the press.