Coral: a term derived from the Greek “Koura halos,” meaning “sea maiden,” referring to ornamental material used since ancient times. It originates from the rigid supports (exoskeletons) of marine invertebrate colonies belonging to Hydrozoa and Anthozoa (Hexacorals and Octocorals). In some genera, these skeletons are fully calcified; in others, they remain as originally composed of organic (horny or proteinaceous) substances; in yet others, the skeletons have a mixed composition (carbonate and organic) due to partial replacement of the organic material by Calcium Carbonate. Density ranges from 2.70 g/cm³ (calcified specimens) to 1.33 g/cm³ (horny specimens), Mohs hardness is 3.5, or 2 in horny specimens, and the refractive index ranges from 1.50 to 1.65.
