Coral reefs form when the calcite skeletons of coral build upon each other.

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Coral reefs form when the calcite skeletons of coral build upon each other.

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This is true. To form their skeletons, coral animals absorb the calcium element from seawater. The calcium is then combined with carbon and oxygen to form calcite. When the animals die, skeletons remain behind them, and eventually, a coral reef begins to build up on them.
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