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
Daniel Pearl through music. Pearl was born in Princeton, New Jersey to Ruth and Judea Pearl. His mother is of Iraqi-Jewish descent and his father is an Israeli of Polish-Jewish descent. His family moved to the Encino district of Los Angeles, when his father took a position with UCLA. His father, Judea Pearl, is a professor of computer science and statistics, and director of the Cognitive Systems Laboratory at UCLA. He has received the Turing Award. The history of the family and its connections to Israel are described by Judea Pearl in the "LA Times" article, "Roots in the Holy Land". Pearl