All pearls are layered with nacre, the gradually thickening coating that covers the initial irritant to form the pearl, and nacre is the same material as the oyster's shell, creating an opalescent sheen known as "mother of pearl."
Nacre is made of two substances, aragonite -- which is translucent, giving the pearl its beautiful lustrous appearance and conchiolin -- which acts as a glue holding the layers of aragonite together.
When nacre, lustre, color, shape and smoothness are all relatively equal, it's the size of the pearl that is the final determining factor of its value.
Mother of pearl is created from the nacre on the inside layer of an oyster shell, but genuine pearls are created inside the oyster.
Lustre is directly linked to the depth of a pearl's nacre, so it's easy to understand why thicker nacre is so desirable.
As a defense against the irritant, the oyster's body secrets a substance called nacre which coats the irritant.
Rend., 188 4, 9 8, p. 1 44) obtained a form which he termed nacre (or pearly) sulphur; the same modification was obtained by Sabatier (ibid., 1885, 100, p. 1346) on shaking hydrogen persulphide with alcohol or ether.
The zone of the external surface of the mantle within the edge secretes a layer formed of prisms of calcite; the rest of the epithelium from this zone to the apex secretes the inner layer of the shell, composed of successive laminae; this is the nacreous layer, and in certain species has a commercial value as nacre or mother-of-pearl.
Thus the growth of the shell in extent is due to additions to the prismatic layer at the edge, its growth in thickness to new layers of nacre deposited on its inner surface.
Perna and pernocchia meant a shellfish which yielded "nacre" or mother-of-pearl.