Cystine calculi are radiopaque.

Cystine calculi are said to be either radiolucent or radiopaque. In the past, contamination of the calculi with calcium has been given as the reason for a radiopaque appearance. However, most cystine stones are pure cystine and contain essentially no calcium. When compared to adjacent fluids and tissues, they are radiopaque because of their higher physical density and their higher effective atomic number.