Yes, if it is an organic reef. However, contrary to the other answer, reefs are built by many liviing organisms, not just corals (cnidarians), and most coral reefs have a large number of other organisms that contribute significantly to building the reef. Modern coral reefs are composed of 40-80% coral, with the other primary component being calcareous algae. In some instances calcareous algae compose almost all of the reef. Many other organisms inhabit the reef and contribute to its build-up such as bryozoans, encrusting molluscs, and even microbes which often cement particles together. Other types of organisms do build reefs too. Oysters build reefs in some modern oceans.