gastrolith
noun
gas·tro·lith ˈga-strə-ˌlith 
: a stone or pebble ingested by an animal and functioning to grind food in gastric digestion
Example Sentences
Recent Examples on the Web Display cases also hold opalized freshwater crayfish gastroliths, or yabby buttons, which are found only in Lightning Ridge’s opal mines. 
Michelle Innis, New York Times, 5 July 2016
Michelle Innis, New York Times, 5 July 2016 Word History
First Known Use
1854, in the meaning defined above
Medical Definition
gastrolith
noun
gas·tro·lith ˈga-strə-ˌlith
: a gastric calculus