Enteroliths are mineral accumulations of magnesium-ammonium-phosphate (struvite) around a foreign object (a piece of metal, pebble, baling twine, hair, rubber) that form round, triangular, or flat stones inside the bowel, usually over the course of multiple years. They form in the large colon of horses, where they can remain for some time until they move and cause an obstruction in the large or small colon, resulting in colic.