Gallstones are small, pebble-liked substances which are formations of the hardened deposits of digestive fluid in the gallbladder. There are 2 types of gallstones: cholesterol gallstones and pigment gallstones. Cholesterol stones are usually yellow-green and made of hardened cholesterol. Pigments stones are small dark stones made of bilirubin.
The causes are not clear. However, it may be due to excessive amounts of cholesterol or bilirubin in the bile and because the gallbladder does not empty completely or often enough.