Cays are made up mostly of?

Answer 1

Cays are islands that are entirely made up of biogenic sediment. Biogenic sediment are the remains of animals and plants.

Answer 2

Cays are mainly composed of coral or other sedimentary material like sand and rock fragments. These small, low-elevation islands typically form on coral reefs or atolls and have a distinctive shape due to the accumulation of these materials over time. They are often found in tropical regions with clear, shallow waters.