It is well - known that certain types of oysters can turn irritants ​ — small fragments of stone, for instance — ​ into lustrous pearls by enveloping them in layers of a secretion known as nacre.
Kad dažu gliemeņu ķermenī iekļūst kāds svešķermenis, piemēram, smilšu graudiņš, dziedzeri ap to sāk izdalīt perlamutru, un tā gliemenē izveidojas pērle.