Gallstones: What are they? When the chemicals in the gallbladder - cholesterol, calcium bilirubinate, and calcium carbonate - are out of balance, gallstones may form. There are two main types of gallstones: Cholesterol gallstones - these may form if there is too much cholesterol in the bile. They are the main type of gallstones in the United Kingdom and United States. Pigment gallstones - these form when the bile has too much bilirubin. They are more common among patients who have liver disease, infected bile tubes, or blood disorders, such as sickle-cell anemia . Gallstones causes and risk factors Experts are not completely sure why some people develop the chemical imbalance in their gallbladder that causes gallstones, while others do not. However, we do know that gallstones are more common among overweight/obese people, especially women. A study revealed that a bulging midriff almost doubles a woman's chances of developing gallstones and the need for surgery