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.
مروارید برخلاف طلا ،‏ نقره یا سنگ‌های قیمتی محصول موجودی زنده است .‏