Actress Mia Farrow has testified that model Naomi Campbell said she got a "huge diamond" from men sent by ex-Liberian president Charles Taylor.
Ms Farrow's testimony directly contradicts Ms Campbell's account that she received two or three stones and did not know who sent them.
Linking Mr Taylor to illegal "blood diamonds" is key to the prosecution's case at his war crimes trial in The Hague.
Mr Taylor denies all 11 charges.
He is accused of war crimes during Sierra Leone's civil war, including using the diamonds to fund rebels.
Giving evidence to the Special Court for Sierra Leone in the Netherlands last week, Ms Campbell said she was given some "dirty-looking stones" after a dinner hosted by former South African President Nelson Mandela in 1997.
But she said she did not know they were diamonds or who the gift was from.
However, Ms Farrow told the court that when Ms Campbell came down for breakfast the next morning, she began speaking even before she sat down.
"What I remember is Naomi Campbell... said, in effect, 'Oh my god... last night I was awakened by knocking at the door and it was men sent by Charles Taylor and he sent me... a huge diamond'," Ms Farrow said.
Ms Farrow said the suggestion that Mr Taylor sent the gift came directly from Ms Campbell, contradicting Ms Campbell's testimony that she did not know who had sent it.
"And she said that she intended to give the diamond to Nelson Mandela's children's charity."
Ms Campbell last week told the court she had given the stones to Jeremy Ractliffe of the Nelson Mandela Children's Fund (NMCF) the next morning, because she wanted the stones to go to charity.
Mr Ractliffe has now handed the gems to police, and on Sunday they confirmed the stones were real diamonds.
After Ms Farrow, Ms Campbell's former agent, Carole White, is due to testify before the court.
Both women were at the breakfast where Ms Campbell is said to have told them about the late-night gift delivered to her room.
Ms White has told prosecutors that Mr Taylor and Ms Campbell were "mildly flirtatious" throughout the dinner, and that she had heard him promise the model a gift of diamonds.
"It was arranged that he would send some men back with the gift," said the notes of an interview prosecutors had with Ms White in May.
Ms White said Ms Campbell "seemed excited about the diamonds and she kept talking about them".
Mr Taylor, 62, was arrested in 2006 and his trial in The Hague opened in 2007.
The former warlord and president of Liberia 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 who murdered, raped and maimed Sierra Leone civilians, frequently hacking off their hands and legs.