"The timing of this dinner is important because if this story ... is true, it places Taylor in possession of at least one rough-cut diamond the month after the rebels came to see him, and the month before [Taylor's] junta received a large shipment of weapons in October," the Open Society Justice Initiative, a human rights organisation monitoring the Taylor trial, told the Reuters news agency.
Taylor denies story
Taylor has called the story of the gift "nonsense" and has denied the charges against him.
They include murder, rape, mutilation, sexual slavery and conscription of child soldiers during wars in Liberia and Sierra Leone, in which more than 250,000 people were killed and where Taylor allegedly armed, trained and commanded the rebels.
His lawyers had filed a motion to delay Campbell's appearance, arguing Taylor's right to a fair trial would be impinged if prosecutors did not provide a summary of her likely testimony beforehand.
The story of the diamond first emerged when Mia Farrow, a US actress, told
prosecutors in a statement that Campbell had told her the morning after the
1997 dinner that Taylor's men had come to her room during the night and presented
her with a rough, or uncut, diamond.
Farrow is due to testify in person on August 9.
Campbell had to be subpoenaed after declining to co-operate with prosecutors.
In July, judges at the court ordered her to appear or face a maximum sentence of seven years for contempt.
In April, Campbell told ABC News she "never received a diamond'' from Taylor.
In May, she told Oprah Winfrey she did not want to be involved in Taylor's case and feared for her safety if she were.
Judges have ordered special security measures for Campbell, saying her fears for her safety and privacy are legitimate because of her high-profile status and the intense media interest in her appearance.
Photographers are barred from taking pictures of her inside the court building and as she arrives and departs.