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Corals are marine animals typically living in compact colonies of many identical individual polyps. The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton. Precious coral or red coral is the common name given to Corallium rubrum and several related species of marine coral. The distinguishing characteristic of precious corals is their durable and intensely colored red or pink skeleton, which is used for making jewelry.

The hard skeleton of red coral branches is naturally matte, but can be polished to a glassy shine. It exhibits a range of warm reddish pink colors from pale pink to deep red; the word coral is also used to name such colors. Owing to its intense and permanent coloration and glossiness, precious coral skeletons have been harvested since antiquity for decorative use. Coral jewelry has been found in ancient Egyptian and prehistoric European burials, and continues to be made to the present day.

At the beginning of the Christian era, there was a great trade carried on in coral between the Mediterranean and India, where it was highly esteemed as a substance endowed with mysterious sacred properties. It is remarked by Pliny that, previous to the existence of the Indian demand, the Gauls were in the habit of using it for the ornamentation of their weapons of war and helmets.

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The origin of coral is explained in Greek mythology by the story of Perseus. Having petrified Cetus, the sea monster threatening Andromeda, Perseus placed Medusa's head on the riverbank while he washed his hands. When he recovered her head, he saw that her blood had turned the seaweed (in some variants the reeds) into red coral. Thus, the Greek word for coral is "Gorgeia", as Medusa was one of the three Gorgons. Poseidon resided in a palace made of coral and gems, and Hephaestus first crafted his work from coral. The Romans believed coral could protect children from harm, as well as cure wounds made by snakes and scorpions and diagnose diseases by changing color.

Various coral types and colors are found in different parts of the world. The best quality red sea corals are found in Italy around the Mediterranean Sea, Red Sea and the sea around Japan. Pink coral is found in a limited quantity in the Mediterranean region, Japan, China and India. The pink Hawaiian coral is also fairly famous. Blue coral is found in the coral reefs in the sea around Philippines. The black coral gem is gathered near the Hawaiian island, Australia, Red Sea, West coast of Africa and other parts of the Mediterranean region.