Coral

In Ovid's canonized tale of Perseus' quest to slay Medusa, the Greek hero is not only credited with defeating the most heinous of Gorgons, but also with discovering coral for the first time: when he finishes using Medusa's head to petrify a sea monster, Perseus notices that Gorgon's blood has hardened nearby seaweeds into wonderful red coral beads. Ovid's mythical prehistory also tells us that Poseidon lived in a palace partially built of coral, and that Hephaestus, the ultimate craftsman, first used coral to build his early inventions. Like the Greeks, the ancient Egyptians also applied coral beads to decoration, and are known to have employed it in royal burial ceremonies. Coral and coral beads even have a long history of use by senior monks in Tibet, and has been labeled one of the seven jewels of Buddhism as described by ancient Sanskrit texts. Coral has been used reverently, by many cultures and since time immemorial, for its intriguing visual characteristics and metaphysical properties.

Nowadays, while fewer people believe that it can protect against evil spirits, the bold visual effect of coral beads is felt by all, and is commonly used to accent skin tones and complete the look of a high-impact outfit. It should be noted that the red coral beads used by modern jewelers is actually markedly distinct from other kinds of coral associated with barrier reefs: coral beads in the jewelry world refers specifically to corallium rubrum, an oceanic strain that is distinguished by its durable, opaque, and bright red skeleton. The incredible softness and porous property of coral (by gemstone standards) also means that it can be effectively dyed. This happens often, and is in some cases merely to accentuate the original colour or to achieve a more uniform surface. Polishing coral beads is also used almost universally to replace the original matte exterior with a high-gloss sheen. Value is determined by surface smoothness, absence of blotches and scratches, and natural colour; while red is by far the most common variety, corallium rubrum, or 'precious coral,' as it is known, also occurs in many different colours- blue being among the rarest.

Chemically, precious coral is remarkably similar to the other organic gemstone giant, pearls. Both are at least 9 tenths calcium carbonate, and both are, of course, provided by underwater life forms. While huge trawling nets have been used to harvest precious coral in the past, today it is collected in a non-destructive and environmentally friendly way: handpicked by divers.

Source: http://www.stonesandfindings.com/jewelry-making-articles/articles/coral