The Pearl, its story, its charm, and its value

by Wallis Richard Cattelle

Adjoining the inner edges of the middle shell plates is the nacreous lining. In this the calcium carbonate takes the same form as the mineral aragonite and is identical with it. As a mass however, the specific gravity is somewhat less, owing to the inclusion of organic matter with the mineral in the shell. This material is harder, finer, more compact, and contains less organic matter than that of which the middle and outer shell is composed.

The lining is constructed of thin waves of transparent calcium carbonate set in animal tissue of great tenuity. This is the mother-of-pearl, and the gem differs from it only in its more or less rounded and independent formation. The plates of which the lining is composed lie almost parallel to the plates of the epidermis. They are bent a little toward the interior at the inner surface of the shell, but the general sectional structure of a shell, cutting from the umbo to the lip, is fairly represented by that stem of the letter X which extends from the right upper to the left lower, the diagonal line representing the middle shell; the horizontal lines at the extremities show the general trend of the epidermis and the nacreous lining. The diagonal trend downward is from the epidermis toward the boss-end of the shell.

The nacreous plates, or mother-of-pearl, unlike those of the middle shell of the nigger-head, cannot be easily separated. On cutting them across the grain they appear as distinct and separate strata and show dividing lines, yet the mass is compact to a great degree. Upon being broken, these strata separate only at the edges, so that the entire set usually breaks diagonally, showing a small strip of the surface of each plate along the broken edge and forming a series of ragged edge steps.

These plates or strata are composed of a great many very thin waves following one upon the other, and thereby producing series of fine, irregular lines upon the surface which, though trending generally in straight lines, curve and twist about as do the edges of water waves, when they run up on the sands of the sea-shore. It is the lapping of these thin transparent waves, and the minute undulations of the layer edges reflecting through the transparent plates, which produce the soft luster peculiar to the linings of the shells and the surface of pearls, and which is known as "pearly."

The wave edges do not usually produce iridescence, but if the waves are very thin and transparent the undulating lines of many under waves following close upon each other appear on the surface, under the microscope, as dark lines when the light is passed through the skin, or silvery lines if the light be thrown upon it from above; to the naked eye this becomes the tempered brilliancy of the pearl's orient. Under the microscope these waves appear to be constructed of minute hexagonal plates or prisms set in animal membrane.

A set of waves forming a plate, when broken at right angles to the trend of the wave, shows under the microscope a rough irregular edge, and the small plates of which they are composed sometimes appear separated individually from the mass though more often they are dislodged in clusters or strips. Broken with the trend of the wave edges, the plate breaks diagonally in steps with undulating edges, which correspond in appearance with the successive underlying waves as they are seen through the surface under the microscope.

Although distinct dividing lines between the plates appear when a sectional cut is made across the grain, there is no indication of a division between the waves which make up the plates, and there is no apparent difference in the structure or compactness at the junction of the plates though a clean division can only be made there. It would appear, therefore, that the plates mark intervals in the process of construction and that the animal tissue is somewhat thicker between the plates than between the waves of which they are composed, where the formative process has been continuous.

In all parts of the shell, the calcium carbonate takes the hexagonal form: in the nacre, as thin waves composed of hexagonal faces, and in the middle shell and epidermis, as plates of hexagonal particles grouped as hexagonal prisms whose terminations form the front and back of a plate. All the parts show a similar plan of construction, i.e., separable plates composed of thinner plates more compacted together, and these in turn of infinitesimal hexagons of calcium carbonate; full plates, component plates, and particles, all alike surrounded by animal tissue.

The shell is built up of secretions from the water in which the oyster lives, made by the mantle, a membraneous covering of the fish. The function of this mantle, in part, is to obtain from the water the elements required and exude it at different parts of its folds in the various forms required for the several parts of the shell. The necessary lime exists in the surrounding water and is supplied sometimes by the calcareous beds upon which the oysters grow, and in other cases by surrounding vegetation.

In all mother-of-pearl oysters and the fresh-water mussel unio, the lining is usually quite thick, but in some pearl-bearing species having small, frail shells, it is, though beautiful, too thin to be of use. In the meleagrina, this nacreous lining lies in the interior of the shell like a congealed pearl wave, the smooth even rim following the curve of the shell about an inch to an inch and a half within the jagged edge of the epidermis, as shown in the Manilla shell illustrated herewith, in which the lip, usually trimmed off for commercial purposes, is preserved. The lining of the meleagrina is not as iridescent as that of the thin shell varieties.