A pearl is an organic gemstone formed within the soft tissue of certain shelled mollusks, such as oysters and mussels. Unlike most gemstones extracted from the Earth, pearls are unique in their biological origin. This article explores what real pearls are made of and how they are formed.
The Composition of Nacre
Real pearls are primarily composed of a substance called nacre, often referred to as mother-of-pearl. This material also forms the iridescent inner lining of the mollusk’s shell. Nacre is a biocomposite, meaning it is made from both inorganic and organic components.
The inorganic part of nacre consists of microscopic hexagonal crystals of calcium carbonate, specifically in the form of aragonite. These tiny crystals are arranged in layers, resembling a brick-and-mortar structure. The organic component is a protein binder known as conchiolin. Conchiolin acts like the mortar, holding the aragonite “bricks” together, providing both flexibility and resilience to the pearl. This layered arrangement of aragonite crystals and conchiolin is responsible for the pearl’s characteristic iridescent luster, known as orient.
How Pearls Are Formed
Pearl formation begins as a protective response by the mollusk when an irritant enters its shell or soft tissue. This irritant can be a parasite, a piece of shell, or even a grain of sand. To neutralize the foreign object, the mollusk’s mantle tissue, which is responsible for secreting shell material, starts to encase the irritant.
The mollusk forms a “pearl sac” around the intruder and continuously secretes concentric layers of nacre around it. This process involves the mollusk depositing the aragonite and conchiolin materials, layer upon layer, effectively isolating the irritant and smoothing its surface. Over time, this continuous layering builds up the pearl, with its size and shape depending on the mollusk species, the duration of formation, and environmental conditions.
Natural Versus Cultured Pearls
Cultured pearls are often questioned as “real” pearls. Both natural and cultured pearls are real, sharing the same composition and biological formation process. They are both created by mollusks secreting nacre around an irritant.
The primary difference between natural and cultured pearls lies in how the irritant is introduced into the mollusk. In natural pearls, the irritant enters the mollusk by chance, without any human intervention. For cultured pearls, a pearl farmer deliberately inserts a small irritant, such as a shell bead nucleus or mantle tissue, into the mollusk. From that point onward, the mollusk’s biological response is identical to that of a natural pearl, secreting layers of nacre to form the pearl.