Cholesterol stones are made mostly of hardened cholesterol, and are usually yellowish-green in color.
Pigment stones are made of bilirubin, and are usually small and dark in color.
Gallstones can be as small as grains of sand or as big as golf balls. Smaller stones are more common, though bigger stones do develop.
"Common Gastrointestinal Problems - Gallstones." American College of Gastroenterology. 13 Jul 2009
"Gallstones." American College of Gastroenterology. 13 Jul 2009
"Gallstones." NIH Publication No. 07–2897 July 2007. National Digestive Diseases Information Clearinghouse (NDDIC). 13 Jul 2009