 Solution

Gallstones contain chiefly cholesterol and/or bile pigment, with variable calcium salts and an organic matrix. They are mixtures with different predominant components, not one compound with a universal formula. The period word cholesterin denotes cholesterol.

Cholesterol-rich stones contain crystalline cholesterol, often with a smaller pigmented nucleus and layers of other material. Mixed stones combine cholesterol, pigment, calcium salts and an organic matrix including mucin. Pigment stones contain much less cholesterol; a major constituent is calcium bilirubinate, related to the bile pigment bilirubin. Minor inorganic constituents can include calcium carbonate and calcium phosphate. Not every visibly dark stone has the same composition, and no fixed percentage follows from the question.^[ref]

The useful modern subdivision of pigment stones is into black and brown types. Black stones commonly contain polymerised pigment with calcium salts; brown stones include calcium bilirubinate and calcium salts of fatty acids, particularly in infected stagnant duct bile. These refinements explain why neither “all stones are cholesterol” nor “all stones are lime salts” is adequate.^[ref]

Chemical examination should distinguish the fractions rather than infer them merely from the external colour. Cholesterol can be extracted with a suitable organic solvent and recognised in recrystallised material; microscopy can show characteristic plate-like crystals. A mineral residue after removal or destruction of organic matter indicates an inorganic fraction, but residue alone cannot identify a particular salt. Layering, facets and colour describe growth and packing and are not chemical analyses. In particular, calcium bilirubinate is not interchangeable with calcium carbonate, and cholesterol is not a bile salt.