The gallbladder is opened to reveal these 4, yellow, multifaceted calculi (stones) that are about 1.5 cm in diameter each. The stones try to leave the gallbladder with the bile but can’t fit through the 0.2 cm opening of the cystic duct. This size difference causes the patient lots of pain and that is the reason for the surgery. The stones are intermixed with a dark brown bile. The color of the bile and stones always varies. The color corresponds to the composition of the stones. Usually yellow stones are made primarily of cholesterol and black/pigment stones are made primarily of of bilirubin.
http://en.wikipedia.org/wiki/Gallstone