There are several kinds of gallstones. Cholesterol stones are the most common type, accounting for over 85% of gallstones in the Western world. Smaller, pigmented stones are less common and they are made up of mostly calcium bilirubinate. Pure stones, however, are almost never found. All gallstones contain variable amounts of bile pigments, cholesterol, calcium carbonate, and apatite, and their core usually consists of bile pigments or mucoprotein, which is secreted by the inner lining of the gallbladder, also known as the epithelium.
The basis of the common cholesterol stone appears to be a precipitation of cholesterol that cannot be absorbed. Cholesterol, which is normally insoluble in water, is made soluble in water when combined with bile. Where there is too much cholesterol secreted, as a result of obesity or diabetes, bile can be supersaturated with cholesterol, forming cholesterol stones. On the other hand, some patients have a reduced bile salt pool, causing the cholesterol stones to form. Gallstones are also a common accompaniment of cirrhosis, as there is often a reduction in the bile salt pool and a decrease in the bile salt content of the bile.
Pigment stones consist of variable quantities of calcium bilirubinate, cholesterol, and calcium carbonate. Pigment stones produce a predominantly brown or black colour, depending on the composition of the stones. These mixed stones are typically found in multiple groups, and form around organisms and inflammatory debris.
Most gallstones form in the gallbladder, but some are formed in the bile ducts, especially after cholecystectomy (removal of gallbladder) or infection. There may also be dietary and genetic factors in gallstone formation. For instance, you are at higher risk for gallstones if you are obese, and women are 3 times as likely as men to develop gallstones.