Keystone is a town in the Black Hills region of Pennington County, South Dakota, United States. The population was 311 at the 2000 census. It had its origins in 1883 as a mining town, and has since transformed itself into a resort town, serving the needs of the millions of visitors to Mount Rushmore National Memorial, which is located just beyond city limits. Among its tourist attractions is the Black Hills Central Railroad, built in 1900 for Black Hills gold. It now operates passenger trains pulled by preserved steam locomotives. Carrie Ingalls spent a significant part of her adult life here, living with her husband David N. Swanzey and his children. Her sister Mary Ingalls lived with them for a while and died here in 1928.