A calculus or, more prosaically, a stone, is any accretion of material within an organ of the body. They are most commonly a collection of mineral salts. Gallstones and kidney stones are the two most common types of calculus.
Although a calculus can cause symptoms, most are asymptomatic, particularly if they are much smaller than the organ they occur in. However, they can cause localized inflammation or, more seriously, can block a duct.
A calculus usually forms because of an excess of certain materials in the diet or the environment. For example, kidney stones are often caused by excess calcium in the diet, or due to a malfunction in the organ that encourages their growth.