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 Note Gravity 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, 3–3⁄16 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 [Illustration: Cross section of an irregular pearl, magnified 80 diameters ] [Illustration: Cross sections of pearls, magnified 30 diameters ] [Illustration: Thin section of mother-of-pearl, magnified, showing sponge borings which traversed the pearl shell ] [Illustration: Structure of conch pearl produced by fracturing, magnified 80 diameters ]