Saltwater whole cultured pearls grow from a piece of what?

Whole cultured pearls, particularly saltwater varieties, grow primarily from a bead nucleus, which is an essential element in their formation. In the cultivation process, a small bead made typically from a material like mother-of-pearl is implanted into the oyster along with epithelial tissue from another oyster. The oyster then secretes layers of nacre (the iridescent substance) around this bead over time, leading to the development of a pearl.

The bead nucleus serves as the central starting point for the pearling process, stimulating the oyster's natural defense mechanism to form nacre. This is distinct from other factors involved in pearl cultivation such as the shell or the water, which play supporting roles in creating the environment needed for pearl formation but do not directly contribute to the structure of the resulting pearl itself. Similarly, while salt is a component of the surrounding marine environment for saltwater pearls, it does not relate to the growth process of the pearl directly. Thus, the bead nucleus is the correct answer, as it is the specific component from which the pearl actually develops.