Pearl or parasite enters a bivalve mollusk and settles inside the shell. The mollusk, irritated by the intruder, forms a pearl sac of external mantle tissue cells and secretes the calcium carbonate and conchiolin to cover the irritant. This secretion process is repeated many times, thus producing a pearl. Natural pearls come in many shapes, with perfectly round ones being comparatively rare. Typically, the build-up of a natural pearl consists of a brown central zone formed by columnar calcium carbonate (usually calcite, sometimes columnar aragonite) and a yellowish to white outer zone consisting of nacre (tabular aragonite). In a pearl cross-section
Pearl River (China) as the Pearl River just north of Guangzhou. The Pearl River is famed as the river that flows through Guangzhou. The Pearl River's estuary, Bocca Tigris, is regularly dredged so as to keep it open for ocean vessels. The mouth of the Pearl River forms a large bay in the southeast of the delta, the Pearl River Estuary, the Bocca Tigris separates Shiziyang in the north, Lingdingyang in the south, and Jiuzhouyang at the southern tip of the estuary surrounded by the Wanshan Archipelago. This bay separates Macau and Zhuhai from Hong Kong and Shenzhen. The Pearl River is so