Next to the nacre is the middle layer or the shell proper. In species of Margaritifera, this stratum is commonly formed of layers of calcareous prisms arranged vertically to the shell surface. External to this middle or prismatic layer is the epidermis or periostracum, the rough outer coating of varying shades, usually yellow or brown. Where the waves are rough, and the bottom hard and rocky, this covering is thick and heavy, to afford greater protection; but where the waters are smooth and gentle, and the bottom free from rocks, Nature—never working in vain—furnishes only thin sides and slight defense. As is the case with the nacre, the prismatic layer and the periostracum decrease in thickness from the hinge to the edge, and the inside lip of the shell shows the gradual union of the three superimposed layers. The two outer layers are formed by the thick edge of the mantle, the remaining portion—or nearly the entire surface—of this organ secretes the nacral layer.
Not only is the interior of the shell made lustrous and beautiful, but this tendency is exerted toward all objects that come in contact with the soft body of the mollusk, either by intrusion simply within the shell, or deeply within the organs and tissues of the animal itself. All foreign bodies—such as small parasites, diatoms, minute pebbles, etc.,—irritate the tender tissues of the mollusk, and stimulate the pearly formation which in course of time covers them. At first the nacreous covering is very thin; but with added layer after layer the thickness is enhanced, and the size of the object increases as long as it remains undisturbed and the mollusk is in healthful growth.
Chemically considered, aside from the nucleus, the structure of pearls is identical in composition with that of the nacre of the shell in which they are formed. Analyses have shown that those from the fresh-water mussels of England and Scotland, and from the pearl-oysters of Australia and of Ceylon, have nearly identical composition in the proportion of about 5.94 per cent. of organic matter, 2.34 of water, and 91.72 per cent. of carbonate of lime.[[70]] The specific gravity ranges from nearly 2 to about 2.75, increasing with the deposit of the nacreous coatings. The following summary by Von Hessling[[71]] shows the results of certain determinations of specific gravity:
| Authority | Specific Gravity | Note |
|---|---|---|
| Muschenbroet | 2.750 | at moderate temperature |
| Brisson | 2.684 | at 14° Réaumur |
| Möbius | 2.686 | 4 fine pearls, weighing 2.396 gms. |
| Möbius | 2.650 | 24 pearls, weighing 6.221 gms. |
| Möbius | 2.336 | 63 brown pearls from Mazatlan, weighing 4.849 gms. |
| Voit | 2.722 | Bavarian pearls, 33⁄16 carats, medium quality |
| Voit | 2.616 | Bavarian pearls, 3⅝ carats, finer quality |
| Voit | 2.724 | Bavarian pearls, 1¾ carats, very fine |
| Voit | 2.578 | Bavarian pearls, gray, with some luster |
| Voit | 2.765 | Bavarian pearls, brown, ranking between good & black |
| Voit | 2.238 | Bavarian pearls, poor black pearls, impure |
Cross section of an irregular pearl, magnified 80 diameters
Cross sections of pearls, magnified 30 diameters
Thin section of mother-of-pearl, magnified, showing sponge borings which traversed the pearl shell