A<unk> (plural calculi), often called a stone, is a concretion of material, usually mineral salts, that forms in an organ or duct of the body. Formation of calculi is known as lithiasis (). Stones can cause a number of medical conditions. Some common principles (below) apply to stones at any location, but for specifics see the particular stone type in question. Calculi are not to be confused with gastroliths.
Calculus 
Ossification
Enema
Cholesterol crystal
Granulation tissue
Neoplasm
Fibrocartilage callus
Granulation