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 of Lao Tzu Pearl of Lao Tzu The Pearl of Lao Tzu (also referred to as Pearl of Lao Tze and previously as Pearl of Allah) used to be the largest known pearl. The pearl was found in the Palawan sea, which surrounds the island of Palawan in the Philippines, and was found by a Filipino diver. It is not considered a gemstone pearl, but is instead known as a "clam pearl" or ""Tridacna" pearl" from a giant clam. It measures 24 centimeters in diameter (9.45 inches) and weighs 6.4 kilograms (14.2 lb). The pearl came from Brooke's Point, Palawan in the Philippines.