Conchiolin

In the normal shell this is all that takes place, but in the case of a mollusc whose interior is invaded by any small source of irritation, such as a borer, or a grain of sand, or other bit of foreign material, a process of alternate deposit of conchiolin and of aragonite goes on upon the invading matter, thus forming a pearl. ❋ Frank Bertram Wade (N/A)

This is deposited by the mollusc in very thin crystalline layers in the horny layers of conchiolin, so that the lining of the shell is built of approximately parallel layers of mineral and of animal substance. ❋ Frank Bertram Wade (N/A)

In the first place, it should be understood that both conchiolin and aragonite are translucent, that is, they pass light to a certain extent. ❋ Frank Bertram Wade (N/A)

Pearls are composed partly of the mineral substance calcium carbonate (chemically the same as marble) and partly of a tough, horny substance of organic nature called conchiolin. ❋ Frank Bertram Wade (N/A)

Interestingly, not all "pearls" are made of nacre, that combination of aragonite (calcium carbonate and conchiolin that is secreted from a mollusk and layered together to form what gemologists call a "nacreous" pearl. ❋ Unknown (2009)