A gallstone is a hard rock-like material that forms inside the gall bladder. The gallbladder is a small pear-shaped organ that stores and concentrates bile. There are two types of gall stones :

Cholesterol stones

They are yellow -green in colour and mainly consists of hardened cholesterol.About 80% of gallstones are cholesterol stones

Pigment stones

They are small , dark stones made of bilirubin.

Gallstones can be large or small, single or multiple.