Corals are made up of the skeletons of many small marine organisms that comprise a colony. These organisms are polyps, each a few millimeters in size. When a new polyp is born, it attaches its skeleton to the colony and thus expands it. These colonies can over the centuries reach several kilometers in length forming coral reefs. Coral reefs consist of calcium carbonate, the material from which the skeleton of each tiny polyp is made. In coral colonies, along with the polyps, some single-celled algae coexist, which photosynthesize and supply food to the polyps, and at the same time secrete pigments that color the colonies, i.e. the corals. Corals can appear in a wide variety of colors such as pink-red, beige, white, as well as black (Yusuri coral). Their hardness is 3.5 and their specific gravity is 2.6.