Richardson died in March after sustaining a head injury while skiing in Canada. She was 45. The Tony Award-winning actress was the wife of film star Liam Neeson and the mother of their two sons.
"This year we lost an angel from our amfAR family. It was one of those random events that leaves you in a state of shock and profound grief. In March, the funny, graceful Natasha Richardson was taken from us," People.com quoted the tearful Stone as saying at the Cinema Against AIDS event at the Cannes film festival Thursday.
"People say, 'Oh, she can't really have been that great or graceful.' She was. 'That funny, charming, sweet and elegant.' She was," Stone said. "She left behind her husband and two stunningly beautiful children and an unfinished mission to help us find a cure for AIDS."
People.com said a short film highlighting the work Richardson did for the charity was then screened at the event.
Stone announced she is donating $50,000 of her own money to the organization in Richardson's memory, and Annie Lennox sang "Talking to An Angel," telling the audience, "That was for Natasha," People.com said.