Piedras Negras, city (1990 pop. 96,178), Coahuila state, N Mexico, on the Rio Grande opposite Eagle Pass, Tex. Founded in 1849, the city grew as an international shipping point. Piedras Negras is a commercial and processsing center for the surrounding agricultural region. Industry includes coal mining and metal-ore furnaces. In 1888, Piedras Negras was renamed Ciudad Porfirio Díaz in honor of the dictator, but the old name was restored after his overthrow.