Planchas de Plata, sometimes called Bolas de Plata is a historic silver-mining district near Nogales, Sonora, Mexico, and a few miles south of the border with the US state of Arizona. Native silver was discovered here in 1736 by Antonio Siraumea, a Yaqui Indian, on the Rancho Arizona of Bernardo de Urrea. Historian Donald Garate believes Urrea's Arizona Ranch to be the likely source of the name of the present US state of Arizona, and he claimed the origin of the name of the ranch was the Basque phrase "aritz ona". Other historians have, however, debated this. For more detail on the etymology of the name Arizona, see Arizona.Toward the end of last October, between the Guevavi Mission and the ranchería called Arizona, some balls and slabs of silver were discovered, one of which weighed more than one hundred arrobas, a sample of which I am sending to you, Most Illustrious Lord. --Captain Juan Bautista de Anza to Bishop Benito Crespo, January 7, 1737.