A pearl is a gem that grows inside a living mollusk. Instead of forming underground from heat and pressure like most gemstones, a pearl grows inside a mollusk, a soft-bodied animal protected by a hard shell, such as a pearl oyster or a freshwater mussel. Pearl Diving in Saudi Arabia: A Lost Heritage.

The process starts when something irritating, such as a tiny parasite or a bit of debris, gets inside the mollusk's soft tissue and cannot be flushed out. The animal responds with the same material it uses to build its shell. Specialized tissue forms a pocket, called a pearl sac, around the irritant and begins coating it with thin layers of a substance called nacre, also known as mother-of-pearl. Nacre contains microscopic crystals of the mineral aragonite arranged in layers with organic material. Layer after layer, the coating builds up, and over time a pearl takes shape.

This defense mechanism is why natural and cultured pearls differ. A natural pearl forms entirely on its own when an irritant enters a wild mollusk by chance, and finding one is rare. A cultured pearl forms with human help: pearl farmers carefully place a small bead or piece of tissue inside the mollusk, return it to the water, and let it coat the nucleus with nacre for a few years before harvest.

The shimmer of a pearl comes from its layered structure. Light reflecting and scattering between countless thin nacre layers produces the soft glow and iridescent play of color, called orient, that makes pearls recognizable. Nacre's layered recipe also makes the gem surprisingly strong for its weight. Pearls occur in many colors, from familiar white and cream to black, depending on the mollusk species and its environment.

The short answer

A pearl forms when a mollusk cannot expel an irritant and coats it with layers of nacre, the same material as its shell. Those stacked microscopic layers give the finished pearl its strength and signature shimmer.

Sources

  1. Gemological Institute of America, "Pearl Description." β€” https://www.gia.edu/pearl-description
  2. Gemological Institute of America, "Pearl Quality Factors." β€” https://www.gia.edu/pearl-quality-factor
  3. Smithsonian Ocean Portal, "How Oysters Create the Ocean's Gems." β€” https://ocean.si.edu/ocean-life/invertebrates/how-oysters-create-oceans-gems

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