Keith Richards stunned music fans at Eric Clapton's Crossroads festival in New York on Saturday night (13Apr13) when he stepped onstage for an unannounced jam session. The Rolling Stones star joined his fellow guitar great towards the beginning of Clapton's hour-long set at Madison Square Garden for blues standard Key to the Highway and rocking number Sweet Little Rock N' Roller. Clapton, who staged the two-night spectacular to raise funds for his Crossroads Centre for substance abuse in Antigua, was also joined joined by John Mayer, Jeff Beck, Robert Cray, Los Lobos and Buddy Guy. And Richards wasn't the only special guest who performed with Clapton on the second night of the festival - Robbie Robertson also joined him onstage to sing I Shall Be Released. The two-night festival also featured appearances from Warren Haynes, Derek Trucks, Gregg Allman, Keith Urban, Vince Gill, Albert Lee, Keb' Mo', Taj Mahal and newcomer Gary Clark Jr. Gill summed up the thrill of the event when he told the sold out crowd, "It's one of the greatest things that ever happened, getting this invitation from Eric Clapton." And Robertson added, "I just want to say it's an honour to be here in honour of Crossroads and Eric Clapton."