You guys are making this harder that what it is. Knowledgeable and articulate people often refer to a "coral reef". The qualifier "coral" in this case would be superfluous if the only reefs were, by definition, created from coral.
There are reefs that are created by organisms other than coral such as sponges and algae. These are clearly reefs and clearly not coral reefs.
Finally, a reef has long been considered by mariners to be any ridge like obstruction at or below the surface of the water. This can be rock, sand, shingle, and yes even coral.
You tropical guys need to get out more. ;-)