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 Thirteenth Pearl The Thirteenth Pearl The Thirteenth Pearl is the fifty-sixth volume in the Nancy Drew Mystery Stories series. It was first published in 1979 under the pseudonym Carolyn Keene. The actual author was ghostwriter Harriet Stratemeyer Adams. "The Thirteenth Pearl" is the end of the original 56-book series published by Grosset & Dunlap. Subsequent volumes were published by Simon & Schuster. Nancy Drew is asked to locate a missing pearl that is unusual, but very valuable. She soon learns that dangerous people are responsible for the theft. They begin harassing her at home and it intensifies when Nancy and her father