Coral is a general term for coelenterates. In our daily life, coral is one of the important organic gems. Everything with strange shape and exquisite appearance is called "coral", and everything with red color is called "red coral". Corals usually include soft corals, gorgonian corals, red corals, stony corals, horny corals, corals, light-colored corals and coral reefs. Some people mistakenly call soft gills and colony anemones "corals". Coral is a kind of marine cylindrical coelenterate named "Coral". In the white larval stage, it is automatically fixed on the calcareous remains of ancestral corals. The chemical composition of coral is mainly CaCO3, which exists in the form of microcrystalline calcite aggregate with a certain amount of organic matter. Most of them are dendritic with longitudinal stripes on them. Each coral has concentric circles and radial stripes in its cross section. The color is often white, but there are also a little blue and black. Gem-grade corals are red, pink and orange. Red is formed because coral absorbs about 1% iron oxide in seawater during its growth, and black is formed because it contains organic matter. It has glass luster to waxy luster, opaque to translucent, and the refractive index is 1.48- 1.66. The hardness is 3.5-4, the density is 2.6-2.7g/cm3, and the density of black coral is relatively low, which is 1.34g/cm3. Sexually fragile. Strong foaming in hydrochloric acid. No fluorescence. So this is an animal paper.