A coral cay is an island built mainly from coral debris, sand and other reef-derived material on the Great Barrier Reef.
Cays form on reef platforms where waves and currents accumulate broken coral, shells and sand. Over time, plants may colonise the newly exposed surface, stabilising it and adding organic material. Many cays are low-lying and vulnerable to storms, erosion and changes in sea level.
Coral cays differ from continental islands. Continental islands are pieces of older geological land separated from the mainland, and they may have hills, streams and soils unrelated to recent reef sediment. The Great Barrier Reef contains both kinds of islands.
The term “cay” is pronounced like “key” in English. A cay is also not the same thing as a coral reef: the reef is the living or geological platform, while the cay is the small island that may accumulate on top of it.