Pearl comes from the excrete nacres by pallium in the bivalves, like mussels and mollusks; and the innermost nacre of a pearl also has the similar luster as the pearl surface.
The birth of a pearl is indeed a great miracle. When the shell, its birthplace, opens to get some breath, some grains of sand, bubbles or other objects will get into the shell that cannot be discharged of at the later time. Thus, those foreign objects will activate the shell's pallium constantly to excrete some nacres to surround them, which is the pearl sacs come from. (Just like some sands get into the eyes, and our eyes will excrete tears to try to make it feel better.) As time passed, the pearl sacs are calculating in layers, and then thick nacres are formed so that a beautiful pearl is come out.
Actually, it is a long process to form a pearl, and very few pearls can be made by the Mother nature. That is why pearls are very precious, and bigger pearls are more precious.
