Pearls are the only 'gem's produced by a living organism. Their formation process, either by natural means or through human intervention, is a response to an injury to the mantle tissue or a protection against an aggression or an intrusion by a foreign body - the exact mechanisms, however, for the glossy nacre formation are unclear.
“Nature's ability to generate so amazingly complex structures like a pearl is impressive and still partially unknown. […] Such a challenge will strongly encourage the community to continue to develop approaches and knowledge to decipher the biophysical and molecular details of biomineralization processes.”
“[…] our fundamental understanding of nacre formation remains limited. Few studies are available that explain the pearl mineralization process as well as the formation of defects […]”
The 2105 study from Université de la Polynésie Française experimentally demonstrated, for the first time, that pearls rotate inside the oyster during their formation - contributing perhaps to their rounded form.