The Amber Skull Bead is the sixty-seventh treasure that can be found in Uncharted 2: Among Thieves.
The Amber Skull Bead is found in Chapter 19 - Siege. When the firefight around you has ceased and you've deemed the area safe, it's time to rush the location of the first treasure, which is in a raised barn on the opposite side of the street from the machinegun nest. More specifically, this area has an RPG that you can acquire (though we recommend against it), and plenty of hay in the background. Along the hay is the treasure in question, so you can simply walk up to it and grab it before proceeding onward.