Calcium carbonate is an abundant biomineral that is of great importance in industrial or geological contexts. In recent years, many studies of the precipitation of CaCO3 have shown that amorphous precursors and intermediates are widespread in the biomineralization processes and can also be exploited in bio-inspired materials chemistry. In this work, the thorough investigation of a urinary stone of a guinea pig suggests that amorphous calcium carbonate (ACC) can play a role in pathological mineralization. Importantly, certain analytical techniques that are often applied in the corresponding analyses are sensitive only to crystalline CaCO3 and can misleadingly exclude the relevance of calcium carbonate during the formation of urinary stones. Our analyses suggest that ACC is the major constituent of the particular stone studied, which possibly precipitated on struvite nuclei. Minor amounts of urea, other stable inorganics, and minor organic inclusions are observed as well.