What is a pearl primarily formed from?

A pearl is primarily formed from nacre, which is a substance produced by several types of mollusks, particularly oysters and some types of clams. When an irritant, such as a grain of sand or a parasite, enters the mollusk's shell, the creature secretes nacre to coat the irritant in layers. This process continues over time, resulting in the formation of a pearl. Nacre, sometimes referred to as mother of pearl, is composed primarily of aragonite or calcite and conchiolin, giving pearls their characteristic luster and unique appearance.

The other options are not relevant to pearl formation. Volcanic minerals do not contribute to the organic process by which pearls are created, and quartz crystals formed in sedimentary layers occur through completely different geological and environmental processes. Lastly, synthetic resin is a man-made material that does not occur naturally in the formation of pearls and cannot replicate the unique biological process that results in authentic nacre production.