Gallstones. Coloured MRI (magnetic resonance imaging) scan of gallstones (red, left) in the gallbladder (yellow), a condition known as cholelithiasis. Gallstones are commonly hard deposits of cholesterol that form due to an imbalance between bile and cholesterol levels. Stones can also arise from bilirubin and calcium salts found in bile. Gallstones can cause sharp pain in the abdomen and may lead to infection. The gallbladder is a small organ that stores bile produced by the liver until it is needed. The bile is released during digestion to emulsify fats. Gallstones are most common in obese people and the elderly.