"Hard corals" refer to a type of coral that has a sturdy skeleton made of calcium carbonate. They are called "hard" because their structure provides strength and support to the coral reef. Full definition
During lunch a snorkel is offered to those who wish to snorkel over coral gardens made up of a variety of hard coral formations and fish life. (wikiaustralia.com)
There you will find the beautiful hard coral gardens that the Outer Reef is famous for. (wikiaustralia.com)
The walls consist of hard coral formations dropping down to far beyond what the eye can see. (twofishdivers.com)