Pearls are concretions in the tissues, of the same material as that which composes the shell-layers, and are usually due to the presence, in the secreting tissue, of some irritating particle or parasite, such as in the tissues of an animal a Trichina becomes covered with a limy cyst. In the mollusk, however, the layers are constantly added to until the pearl reaches a considerable size.
When it becomes so large that the valves of the shell cannot close, the mollusk soon dies and the pearl may be washed away and lost. If the pearl escapes from the tissues during the life of the animal it may become cemented to the inside of the valves; or pseudo-pearls may be formed by the mantle over projections from the inside of the valves.
Some of the concretions partake of the nature of the epidermis, are brown or yellow, without lustre. Most freshwater pearls when sawn in two, show an aggregation of prismatic shell substance radiating from a central point, which alternates with concentric epidermal layers and is externally covered with a layer of true pearl. If the last is thick, clear and iridescent the pearl is valuable, if not, worthless.
The common notion of a pearl being pearly throughout, is mostly incorrect. Disease may set up an irritation which will cause shelly concretions to form in the tissues of the mollusk. These are usually small and irregularly shaped and in the pearl mussel are most frequent in valves. such are called sand-pearls and are mostly used in embroidery and cheap jewellery.
In these cases the pearl lies in the substance of the mantle or tissues, but it may, with increasing size, work out into the cavity of the shell outside the mantle. It is then apt to become attached to the inside of the shell.These are less valuable because less regular in shape and iridescence than free pearls.
In any case the matter of which the pearl is composed is secreted at the expense of the shell, so that it is not strange that a shell containing two good-sized pearls is always recognizable and it is seldom that a shell of perfectly regular shape contains a pearl.
The fishers say that three characters externally show the presence of the pearls. 1st grooves or ridges form the beaks to the margin. 2nd a kidney-shaped outline. 3rd the asymmetry of the valves with regard to the median vertical plane of the animal.