Arequipa is famous for its stunning architecture, which is made of two types of local volcanic rock called sillar. The white sillar is soft and has been used since the 16th century to build many of Arequipa's most famous buildings and structures, such as the Santa Catalina Monastery. The red sillar is a harder and rarer stone, used mainly as an accent in contrast to the white sillar. The color of the stone comes from the iron oxide it contains. Both types of sillar give Arequipa its unique appearance and are an important part of the city's cultural heritage.