Possible Clues:

• Atoll's composition
• What's in mine are shortly all that's in 13 across
• Material from reef originally surrounded by blackish rock
• Animal forming reef
• 30th anniversary gift or Ontario rapids
• Are shortly in the fuel from the bottom of the sea
• Tiny animal forming reef
• Reef component
• Losing a right to enclose sea-life
• Snorkeler's milieu