It was Fawkes tail feathers. It was told to Harry by Dumbledore in the Goblet of Fire. He told him that was why Voldemort couldn't use his wand against him while dueling. :) I guess Voldemort had his wand or possibly wormtail. I reread the books every once in awhile to see if I have found any new clues. :)
Thanks. I thought that they had mentioned it. That gives Harry an extra connection to Fawkes. (The dueling problem had to do with them having tail feathers from the same phoenix -- I just wasn't sure that it was indeed Fawkes, but knowing Rowling, it just made sense that it was.)