Stones may be described as Primary stones (usually brown pigment stones), which form in the bile ducts Secondary stones (usually cholesterol), which form in the gallbladder but migrate to the bile ducts Residual stones, which are missed at the time of cholecystectomy (evident < 3 years later) Recurrent stones, which develop in the ducts > 3 years after surgery In developed countries, > 85% of common duct stones are secondary; affected patients have additional stones located in the gallbladder.