Final answer:
The material unique to the African Swahili coast used in the Great Mosque of Kilwa is coral, which was utilized to construct the mosque and became a significant architectural feature in urban Swahili design.
Explanation:
Coral is the material unique to the African Swahili coast used in the construction of the Great Mosque of Kilwa. The mosque was built with coral blocks and became a defining feature of urban Swahili architecture during the 11th-18th centuries. Coral blocks were a common building material due to the abundance of coral reefs along the Swahili coast.
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