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
The Pearl and the Wave "The Pearl and the Wave" as "the most perfect painting of the nude" in the 19th century. Cox identified some features in the painting which he described as "grace of attitude", the well-rounded but slim body of a young woman, the visible dimple in the shoulder, the "savoring of subtle line", the "loveliness of the color", the "solid yet mysterious modelling", and the "perfection of delicate surface". Cox believed these features make this painting what he calls "a pure masterpiece". Art historian Bailey Van Hook identified "The Pearl and the Wave" as one of the examples of nude paintings where