Model Naomi Campbell has testified that she was given some "dirty-looking stones" after a 1997 dinner attended by ex-Liberian President Charles Taylor.
Speaking at Mr Taylor's war crimes trial in The Hague, she said late that night, two unidentified men appeared at her room and gave her the stones.
However, she had no proof that the stones were diamonds or came from Mr Taylor, as a fellow guest suggested.
Linking him to illegal "blood diamonds" is key to the prosecution's case.
Mr Taylor is accused of using illegally mined diamonds to secure weapons for Sierra Leone's RUF rebels during the 1991-2001 civil war - a charge he denies.
Prosecutors say that from his seat of power in Liberia, Mr Taylor also trained and commanded the rebels.
The rebels were notoriously brutal, frequently hacking off the hands and legs of civilians.
Ms Campbell, who was late appearing in the courtroom, swore on a Bible before beginning her testimony.
She said that in 1997 she had attended a charity dinner in South Africa hosted by Nelson Mandela at his presidential mansion in Pretoria.
Other guests at the event included the actress Mia Farrow, Ms Campbell's former agent Carole White and Mr Taylor.
Ms Campbell said she was sleeping in her room later that night when there was a knock at the door.
"Two men were there and gave me a pouch and said: 'A gift for you'," she said.
The men did not introduce themselves. She said she put the pouch next to her bed without looking inside it - something she claimed was not unusual since she frequently receives gifts from admirers, and went back to sleep.
"I opened the pouch the next morning when I woke up... I saw a few stones in there, they were very small dirty-looking stones," she told the court. There was no explanation and no note, she said.
"The next morning at breakfast I told Ms Farrow and [her former agent Carole White] what had happened, and one of the two said, 'That's obviously Charles Taylor', and I said, 'I guess that was'."
She said that before meeting him, she had not heard of Mr Taylor or of Liberia, adding that she never saw him again and had never asked him whether he had been the source of the gift.
Ms Campbell said that at the time she was not aware of any laws on unprocessed diamonds.
She said she gave the stones to Jeremy Ractliffe of the Nelson Mandela Children's Fund because she wanted them to go to charity, and that when she spoke to Mr Ractliffe on the telephone in 2009, he said he still had them.
In a letter presented in court by the defence, the Nelson Mandela Children's Fund said it had never received a diamond or diamonds from Ms Campbell or from anyone else, a denial a spokesman for the fund repeated on Thursday.
"The point I need to communicate today, on behalf of the board of trustees, is that the Nelson Mandela Children's Fund is not in receipt or possession of a diamond," spokesman Oupa Ngwenya said.
Allegations that the uncut gemstones were given to Ms Campbell emerged in a statement by Ms Farrow.
So-called blood diamonds are stones mined in areas controlled by rebel armies, and used to fund their violence.
Ms Campbell was a reluctant witness and was subpoenaed to appear by prosecutors or risk contempt of court charges.
During her testimony, the prosecutor said Ms Campbell was answering questions before they were finished, and asked her if she was nervous.
"No, well, I didn't really want to be here. I was made to be here," she answered. "So, obviously I'm just like wanting to get this over with and get on with my life. This is a big inconvenience for me."
'Fear for family'
She said she had previously denied having the stones as she feared for her family because Mr Taylor was "someone I read on the internet has killed thousands of people, supposedly".
In April, she told ABC News in the US that she "never received a diamond" from Mr Taylor and did not want to talk about it.
Later, she told US talk show host Oprah Winfrey that she did not want to be involved in Mr Taylor's case and feared for her safety if she were, although her management company later released a statement confirming she would attend "to help clarify events in 1997".
The trial is in the defence phase, with only a few witnesses remaining to testify. The prosecution rested in February 2009 after calling 91 witnesses, but obtained special permission to re-open their case to present new evidence.
Mr Taylor was arrested in 2006 and his trial at The Hague opened in 2007.
Mr Taylor has pleaded not guilty to 11 charges, including war crimes and crimes against humanity, at the UN-backed tribunal.
Tens of thousands of people died in the interlinked conflicts in Sierra Leone and Liberia.