The history of Diamond (Diamond City) rightfully belongs to Eureka County. The U.S. Post office listed Diamond as White Pine County with Chas. Morrill appointed postmaster and the postal designation immediately rescinded. Stanley Paher, Nevada Ghost Towns and Mining Camps, states that Diamond was located at a point 11 miles north of the town of Eureka.
Both Bancroft and Fox (See Resources below) list Diamond Mountain as a settlement of White Pine County. Fox lists both Diamond and Diamond Mountain as in White Pine County with the population of Diamond as 52 and Diamond Mountain as 37 and the mailing address Diamond.
Resources:
Bancroft (Hubert Howe Bancroft, History of Nevada 1540 - 1888, first published in 1890.)
Theron Fox, Nevada Treasure Hunters Ghost Town Guide. Fox lists over 800 places in Nevada that were active in 1880. Many have long since gone. Fox also has an 1881 fold-in map. There may be some error in the map, as many of the places were placed by description. Fox states "the description is liable to vary according to how tired the map-maker was at the end of his journey"