What characterizes a natural pearl?

A natural pearl is primarily characterized by its formation process, specifically that it occurs without any human intervention. These pearls develop in mollusks, such as oysters and mussels, as a response to an irritant that gets inside their shells. The mollusk secretes layers of nacre, which is the same material that lines the inside of the shell, eventually forming a pearl. This natural process is distinct from other types of pearls, such as cultured pearls, which are formed when humans intentionally introduce an irritant into the mollusk to stimulate pearl production.

The other choices reference various alterations or processes that do not apply to the definition of a natural pearl. Dyed pearls, for instance, have undergone color enhancement treatments, while man-made processes or chemical treatments indicate human involvement that disqualifies them from being classified as natural. Understanding this distinction is crucial for identifying and working with pearls in jewelry and gemology.