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
Parable of the Pearl writings. Pope Pius XII used the phrase to describe virginity. The pearl itself is a beautiful, single entity, formed through suffering in the heart of the oyster (in the same way that believers endure lack of wealth or comfort) and like the Church, will be put on display in a coming day. Unlike precious stones which must be cut and polished to reveal their clarity and beauty, the pearl is perfect as it comes from the oyster. A version of the parable also appears in the Gnostic Gospel of Thomas (Saying 76): This work's version of the parable of the