1997: Elected Liberian president
2003: Arrest warrant issued, steps down, goes into exile in Nigeria
2006: Arrested, sent to Sierra Leone
2007: Trial opens in The Hague
Profile: Charles Taylor
Q&A: Trying Charles Taylor
"For avoidance of doubt, she is not being accused of any wrongdoing and is not on trial."
Prosecutors want to know whether Ms Campbell received diamonds from Mr Taylor at a reception hosted by Nelson Mandela in South Africa in 1997.
A spokesman for the special court for Sierra Leone previously said Ms Campbell had denied receiving the gems and refused to talk to prosecutors.
In an interview on the Oprah Winfrey Show, Ms Campbell neither confirmed nor denied that she received gems, instead saying: "I don't want to be involved in this man's case. He has done some terrible things, and I don't want to put my family in danger."
US actress Mia Farrow, who Ms Campbell allegedly told about the gift, may also testify.
Mr Taylor has pleaded not guilty to 11 charges at the UN-backed tribunal.
It has spent more than two years hearing the case.
Mr Taylor, 62, is suspected of selling diamonds and buying weapons for Sierra Leone's RUF rebels, who were notorious for hacking off the hands and legs of civilians during the 1991-2001 civil war.
Tens of thousands of people died in the interlinked conflicts in Sierra Leone and Liberia.