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 Hidden Treasure appears later (Saying 109), rather than immediately preceding, as in Matthew. However, the mention of a treasure in Saying 76 may reflect a source for the Gospel of Thomas in which the parables were adjacent, so that the original pair of parables has been "broken apart, placed in separate contexts, and expanded in a manner characteristic of folklore." In Gnostic thought the pearl may represent Christ or the true self. In the Gnostic Acts of Peter and the Twelve, found with the Gospel of Thomas in the Nag Hammadi library, the travelling pearl merchant Lithargoel is eventually revealed