What is a bead nucleus?

A bead nucleus refers specifically to a small, typically round object that is used as the core around which a cultured pearl forms. This nucleus is usually made from a material such as a freshwater mussel shell, although other materials can be used as well. When an irritant, such as a bead nucleus, is introduced into an oyster or a mussel, the mollusk secretes layers of nacre around it over time, leading to the formation of a pearl. This process is fundamental to cultured pearl production, distinguishing it from natural pearls that form without human intervention.

The other options do not describe bead nuclei accurately. For example, gemstones that form in layers would pertain more to their natural formation processes rather than a specific item used in pearl cultivation. Similarly, the term "type of gemstone cut" relates to how gemstones are shaped and faceted, while an ornamental bead made from glass does not pertain to cultured pearls at all. Thus, the description of a bead nucleus aligns precisely with the practices of pearl cultivation, making it the correct choice.