This cross-cut section of a  cultured pearl shows the various layers of a tissue-nucleated freshwater pearl. Freshwater pearls produced in China are traditionally “tissue nucleated”. The nuclei of the host mussel consist of a small, approximately 1-3mm, piece of mantle tissue obtained from a donor mussel and inserted into its soft body. The presence of this foreign body stimulates the pearl sac formation and the development of a pearl. Tissue-nucleated freshwater pearls are the closest in composition to a natural pearl since they are made primarily of concentric layers of 100% solid crystalline nacre. Conchiolin, an organic protein produced by the mollusk, functions as a base coat and is generally the first layer deposited by the pearl sac. It acts as a form of glue or adhesive and surrounds the bead or tissue nucleus. Alternating layers of conchiolin and crystalline nacre will continue to form causing subsequent nacre layers that are produced to bind together.