The order did not say when Campbell, along with and actresses Mia Farrow and Carol White, will be called. Taylor is accused of stoking civil war in neighboring Sierra Leone partly to exploit its mineral resources and diamond fields.
Campbell is alleged to have received the diamond at a 1997 reception in South Africa.
Defense attorneys argued that prosecutors were introducing evidence too late, nearly 1-1/2 years after they closed their case.
Judges ruled Wednesday the prosecution only learned of the incident a year ago and it was relevant to the case.
