The Keystone Building, built in 1923, is 1 of 2 Stolp Island works by Elmslie. It is a relatively rare example of the Prairie style used for an office building. This style, noted for its horizontal emphasis and decorative applications, is typically found in houses rather than commercial office buildings.
Faced with an essentially triangular piece of land, Elmslie designed the Keystone building 115 feet deep at the north end but just 25 feet deep on the south end. The former Aurora Silver Plate Manufacturing Company factory was successfully incorporated into the eastern portion of the Keystone Building.
The hallways and most of the offices have the original quarter-sawn oak trim and doors. On the exterior, highly refined Prairie School decorative elements are located in the arched screen over the central entryway, in the bronze light fixtures, in the cornice and in the horizontal banding that links the windows at each floor. Ironically, there are no keystones (the centerpieces of masonry archways) in the Keystone Building.