Man has not always know the origin of pearls. The Camden, Britannia (1551-1623) says:
The rivers of Conway, Wales, breed a kind of shell which being [im]pregnated with dew, produces pearls.
How are pearls made Schroeter, treating this subject in 1779 writes:
I would be long-winded if I were to repeat all the opinions which I have heard on the origin of pearls, some of which are tasteless and laughable. I will quote the two newest opinions but I make the assertion that the theory of Herr Chemnitz holds the most worth.
According to his theory, the pearls are protection against hostile attack, and healing-plaster for wounds. The objection to this, that one also finds pearls in the animal itself, appears to me not to be as important as is believed.
If the mussel-animal has the intelligence to know that it is injured, and then works to overcome it, then it seems to me not unreasonable to believe that the animal could have one more pearl on hand to lay down in the area of the wound.
What, then, is the pearl-material? All believe that it is at least the foundations-material of true pearls, which is used by the animal to protect itself when it has no true pearls on hand. I have seen a common oyster, in which half of the inside of one shell was covered with a club-shaped swelling, which is hollow in the middle. The animal could check promptly any hostile creature.
According to Chemnitz's theory, the pearls are protective material against fatal wounds, and healing plaster of previous wounds.
Here I must take leave of Herr Pastor Chemnitz's theory on the origin of pearls. If we accept that pearls are preservation material and healing plaster, then it is to be expected that undamaged and unpunctured shells are in all probability unfruitful of pearls. There are, on the contrary, shells which have suffered great injury, and the probability of them containing a pearl is just the same. How desirable it would be, if one could, by this characteristic, confirm or refute the existence of a pearl.
"The pearl is originally a round, very small, clear piece of mother-of-pearl which is secreted by the animal and built up in the same manner as the shell. It is held onto the shell by a slice of the skin."
So much appears to be right:
"The internal film of the shell, including the pearls, consists of one composite layer, and the pearl itself consists of built-up layers. The pearl originates, though, from the finest elements, which are rarer than those of the shell. This matches Herr Chemnitz's theory, as he states that the animal gives its best material to the pearl, so that it may be very strong, in order to be a plaster in the wound, and prevent any attack from foreign bodies."
How are pearls made Schroeter continues:
The position of the pearls is also variable. They are either fixed in the shell, or they lie free in the animal, without any fastening. In this latter case, according to Herr Hofmedicaus Taube, they lie between the skin of the so-called 'beard' of the mussel or, just as often, are found lying between the inner skins of the animal. Could it be, that the pearl grows in the shell, sitting quite firmly, so that the hidden underside is not ripe? pearls, however, are found in this condition, lying free in the shell. Whether it is right that the pearl leaved its site in the mussel shell when it is ripe, I can neither confirm nor refute. I lack personal experience and new evidence, and the evidence of the older writers is, in this case, not firm.
How are pearls made Schroeter
Mostly, our river-mussels contain only one, or at best, two, pearls, although some examples do contain more.
We can certainly say of our freshwater mussels, that they do not approach the mussels described by Ameritus Vespucius and Caspar Morales, the former having found mussels with 130 pearls, and the latter mussels with 120.