The pearl oysters and top snails or Trochidae create between their body and shell layers of mother of pearl to smoothen and harden the inside of the shell and to protect their body. These sturdy inner layers of calcium carbonate display a silky luster and some, such as those of the abalone, produce a highly attractive iridescence
Coral is an organic gemstone which comes from hardened skeletons of marine animals. In nature, it comes in shades of black, blue, red and white. As coral reefs have become endangered ecosystems, some type of Chinese white coral is cultivated in the inland for commercial and artistic purposes. As a consequence and to meet the modern taste most cultured coral is colored.