Coral has long been used for adornment by human societies. It is actually the calcified skeletons of coral branches formed by tiny polyps who secrete the calcium carbonate that make up the structure. Red coral (ox-blood, dark red) is considered the most valuable, although angel skin pink coral (no trace of orange) is also highly desirable. Coral can be bleached, dyed, and/or treated with a plastic resin or other type of treatment to reduce porosity and improve the surface polish. Natural, untreated coral is rare because it has been extensively farmed over the years from the shallow seas where it is found. Since coral grows slowly, it has not been able to replace the harvested material quickly and so has become more scarce over the years. Be aware that there are many coral imitations in the marketplace.