Possible Clues:

• ___ reef: warm, shallow salt-water area
• A number take exam, naturally it's hard
• Pink and black material will be about right
• Deep pink 11 across
• After a century, in a manner of speaking, it might make an island
• Reef-forming skeletons
• Reef deposit
• Hard pink substance formed by skeletons of tiny sea creatures
• Key feature?
• Cay composition