What is the middle of a pearl?

The middle of a pearl is called the nucleus, which is typically a small, round object made of calcium carbonate or an organic material around which the pearl forms. As an irritant, it triggers the oyster or mollusk to secrete layers of nacre, or mother-of-pearl, which gradually builds up around the nucleus, creating the pearl's lustrous surface. The composition and structure of the nucleus significantly influence the pearl's size, shape, and overall quality.