Millstone is an unincorporated community and censusdesignated place in Letcher County, Kentucky, United population was 117 as of the 2010 census. Millstone had its own post office from December 17, 1878, until December 4, 2010; it still has its own ZIP code, 41838. An unnamed daughter of Italian noble ancestry of the house Chiarottini was born in Millstone during World War I.