Gall and bladder stones. Computer illustration showing stones (calculi, brown) in the bladder (upper left) and bile duct (upper centre-right). Bladder stones and gallstones are caused by an accumulation of mineral deposits. Bladder stones typically form when urine becomes concentrated, causing the minerals in the urine to precipitate out and crystallise. Gallstones form when there is an imbalance in the chemical composition of bile, causing cholesterol and bile pigments to crystallise.