Actress Mia Farrow and Campbell's former agent Carole White both contradicted the model's account that she did not know who gave her a present of "blood diamonds".
All three were present at a party in September 1997 hosted by Nelson Mandela in Pretoria, South Africa.
Giving evidence to the Special Court for Sierra Leone in The Hague, the Netherlands, Ms White said Campbell was flirting with Taylor during the dinner.
She said: "Naomi was very excited and told me 'He's going to give me some diamonds'."
She said Taylor was nodding in agreement and "they were being charming to each other" and "mildly flirting".
According to Ms White, two men brought the gift to the guest accommodation later that night.
Ms White said: "They took out a quite scruffy paper and they handed it to Miss Campbell and said 'here are the diamonds'.
"She opened them and showed them to me. They were quite disappointing because they weren't shiny."
She said she knew taking diamonds out of the country was illegal, adding: "I recall telling Naomi the next morning that we shouldn't take these diamonds and we should actually do something good with them and give them to the children's charity."
The next day Campbell handed the diamonds to Jeremy Ratcliffe, the former head of the Nelson Mandela Children's Fund, a man she described as a "trusted friend".
Ms White said: "He was quite shocked. He looked quite horrified. He didn't want them."
Earlier, Farrow also contradicted Campbell's account of what happened.
She said when the model came down to breakfast the night after the party, she told the guests an "unforgettable" story.
Farrow said it was the model, not herself or Ms White, who claimed the diamonds were a gift from Taylor.
Farrow told the court: "She said that in the night she had been awakened, some men were knocking at the door.
"They had been sent by Charles Taylor and they were giving a huge diamond.
"And she said that she intended to give the diamond to Nelson Mandela's children's charity."
Farrow said: "Miss Campbell entered the room. She was quite excited and said in effect, 'Oh my God, in the middle of the night I was awoken by knocking at the door and it was men sent by Charles Taylor and he sent me a huge diamond."'
Under questioning from defence lawyer Morris Anyah, Farrow said: "I also discussed it with my children and they remember exactly the same thing."
Farrow's three children, Matthew Previn, Malone Farrow and Ronan Farrow, were also in South Africa, at the time aged about 17, 12 and 10.
The actress did later concede that she may not have heard the term "huge".
She said: "My recollection might not be accurate on the size part. She might have said large, she might have just said a diamond, it might have been a huge diamond."
Farrow was asked about her "preoccupation" with Africa and achieving justice.
She said: "If indeed he is guilty of the crimes he is accused of, I am gratified that he is in this procedure, involved in a procedure that will bring him to justice."
Taylor, the former president of Liberia, is accused of war crimes during Sierra Leone's civil war, including using diamonds to fund rebels.
He denies 11 charges, including murder, rape, sexual slavery and recruiting child soldiers.
Defence lawyer Courtenay Griffiths QC said Ms White's had a "very powerful motive for lying".
The agent, who is suing Campbell for breach of contract, said the model owed her about 600,000 US dollars (£375,935) in lost earnings over the past two years.
Mr Griffiths said: "I suggest that you don't come to this court with clean hands. You come with an agenda. I suggest that your motive in lying about Naomi Campbell is to provide you with ammunition for use against her in the law suit."
Ms White replied: "That's not true."
She also described as "pure coincidence" the fact that Farrow's statement to the prosecution came exactly two weeks after Ms White launched her legal action against the model.
Griffiths also asked Ms White about an alleged "blood diamond party" in her offices.
She was shown photos on Facebook posted by one of her booking agents, Annie Wilshaw, including a photo in which she herself appeared.
The caption for the photographs at the party, held last Thursday, read "blood diamond night".
Ms White said the party was for the opening of a new modelling house and nothing to do with blood diamonds, adding: "I'm not crass enough to arrange a party like that. That would be absurd."
Ms White said she did not know how the two men who brought the diamonds could have got past security into the presidential compound.
And she conceded the men had not said they were sent by the Liberian president.
The case was adjourned until tomorrow when Ms White's evidence will continue.
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