The main substance in a pearl is calcium carbonate, primarily in the form of aragonite or a mixture of aragonite and calcite. Pearls are formed by mollusks, such as oysters, as a defense mechanism against irritants, like parasites or foreign objects. When an irritant enters the mollusk, it secretes layers of nacre, a combination of calcium carbonate and conchiolin, which accumulates over time, creating a pearl. The beauty and luster of pearls make them highly valued in jewelry, leading to various types and colors depending on the species of mollusk and the conditions in which they form.