Cultured pearl is calcium carbonate and a fibrous protein called conchiolin. As the nacre builds up in layers of minute aragonite tablets, it fills the growing pearl sac and eventually forms a pearl. Natural pearls are formed by nature, more or less by chance. On the other hand, cultured pearls are human creations formed by inserting a tissue graft from a donor oyster, upon which a pearl sac forms, and the inner side precipitates calcium carbonate, in the form of nacre or "mother-of-pearl". The most popular and effective method for creating cultured pearls are made from the shells of freshwater river mussels
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