The giant clam (Tridacna gigas) is also known as Ōgikai (fan clam) or Shako. It is the world's largest bivalve. It inhabits the shallow waters of coral reefs in tropical to subtropical regions and is the largest species of bivalve. This includes the great clam. It was named for its appearance, which resembles a fan. Giant clams are a rare organic gemstone, white as jade, and a Buddhist sacred object. Giant clams, pearls, coral, and amber are known as the four major organic gemstones. In Chinese Buddhism, it is revered as one of the seven treasures along with gold, silver, lapis lazuli, agate, coral, and pearls. The giant clam is one of the seven treasures of Buddhism, and it is said that chanting the nenbutsu (Buddhist invocation) while using a giant clam rosary will bring twice the merit. It is said to calm the mind, be effective against insomnia, and is good to wear for a long time. It is said to heal mental fatigue and be good for visual impairment, and it is also said to have the power to improve fortune, maintain health, and ward off evil.