Possible Clues:

• Hard substance formed from skeletons of tiny sea creatures
• Marine formation
• Reef buildup
• Orangey-pink
• Leader of Commons' test for marine growth
• Word with reef or snake
• Reef-forming sea life
• Carbon said to be in hard material
• A Beethoven symphony, not hard rock, perhaps
• This build-up in the water takes a century, in a manner of speaking