Indiana lived in Coenties Slip, a neighborhood in Lower Manhattan, from 1956–65. There he turned to the local environment for inspiration, sketching marine scenes and taking designs and names on boats as a basis for ideas for his paintings.
Hardrock and its companion painting Bardrock (1961) were inspired by an inscription on the side of a barge; both are a variant of Trap Rock, the name of a company whose barges Indiana often watched from the Slip.  The star was featured on the funnel (or smokestack) of the tugboat Newport, a boat that Indiana can be seen drawing in a 1957 photograph by Jack Youngerman.
 Susan Elizabeth Ryan, Robert Indiana: Figures of Speech (New Haven: Yale University Press, 2000), p. 142.