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 Kuwait at the Victoria and Albert Museum in London, which closed on 19 January 2014. Its owners have made claims that it is the same pearl that is depicted in the portraits of Mary Tudor. The Pearl of Kuwait outweighs La Peregrina by a small amount. The pearl is currently on loan to the Smithsonian, who make no claims of its history or provenance. The Smithsonian does say that it is exceptional, that is weighs 64.4 carats, that its setting is 19th century in style, and that the pearl itself "recalls" those that originate from the Persian Gulf. Pearl of Kuwait