What material is used for the exterior surface of Frank Lloyd Wright's Gammage Auditorium?

Frank Lloyd Wright's Gammage Auditorium features an exterior surface primarily made from concrete. This choice is notable because concrete allows for both functional and aesthetic advantages in architectural design.

Wright was known for his innovative use of materials and his designs often harmonized with the environment. In the case of Gammage Auditorium, the concrete provides durability and a strong structural integrity necessary for a large performance space, while also facilitating the sweeping, horizontal lines and forms characteristic of Wright's style. The use of concrete also complements the desert landscape surrounding the auditorium, blending the structure with its environment, which was an important aspect of Wright’s philosophy of organic architecture.

Additionally, concrete was a practical material for the climate of Arizona, allowing for the thermal mass needed to help regulate indoor temperatures. This choice reflects Wright's intention to create a building that not only serves its purpose as a cultural venue but also interacts harmoniously with its setting. The other materials mentioned in the options, such as ceramic, glass, and stone, do not play a significant role in the external facade of the auditorium and therefore do not align with Wright's design for this particular structure.