Which type of coral is recognized as a variety of conchiolin coral?

Black coral is recognized as a type of conchiolin coral due to its unique structural composition. Conchiolin corals are characterized by their skeletal structure that includes conchiolin, a protein that serves a similar role as collagen in other organisms. Black coral has a hard, external skeleton made predominantly of this protein, giving it distinct properties compared to other types of coral.

Additionally, black coral is well-known in jewelry making and for ornamental uses because of its beautiful color and durability. Its composition differentiates it from the other types of coral listed, such as red, blue, and green coral, which do not share the same conchiolin characteristics and are composed of calcium carbonate instead. Therefore, black coral stands out as the correct answer due to its classification and structural properties within the group of conchiolin corals.