 Named for the state nickname, "Gem State," the county was established on March 15, 1915, partitioned from Canyon County and Boise County.Fur trappers were in the area as early as 1818, and Alexander Ross explored Squaw Creek in 1824.Prospectors and miners moved through the county in 1862 en route to the gold rush in the Boise Basin around Idaho City, and by the next year irrigation began along the Payette River.