form
or beautiful its color. This luster is due to the structural
arrangement of the surface as well as to the quality of the material.
The nacreous material forming true pearls, and likewise
mother-of-pearl, is commonly deposited in irregular tenuous layers,
very thin and very small in area compared with the surface of the
pearl. These laminae overlap one another, the surfaces are
microscopically crumpled and corrugated, and the edges form serrated
outlines. The greater the angle which the laminae form with the
surface, the closer will be these serrated outlines, and where the
plane of the exterior lamina is parallel with the plane of the surface
the lines are not present. This arrangement causes the waves of light
to be reflected from different levels on the surface, just as in a soap
bubble, and the minute prisms split the rays up into their colored
constituents, producing the chromatic or iridescent effect.
The
cause is wholly mechanical, and an impression of the surface made in
very fine wax shows a similar iridescence. Also, if a piece of
mother-of-pearl be immersed in acid until the surface lime or shelly
matter is dissolved, the pellucid membrane shows the iridescence until
it is so compressed that the corrugations are reduced. About two score
years ago an Englishman invented steel buttons with similar minute
corrugations producing pearly effect, but the manufacture was
unprofitable, owing, principally, to their liability to tarnish.
In the shells of some mollusks—as the edible oysters (Ostrea) or the giant clam (Tridacna),—there
is almost a total absence of the crumpled corrugated laminae, and,
consequently, there is little luster. In others the nacre is of better
quality, resulting in superior orient, and it probably reaches its
highest degree of perfection in the pearl-oyster (Margaritifera).
As
the curvature of the surface of pearls is greater, and the minute
striae are more numerous, than in ordinary mother-of-pearl, it follows
that the iridescence is likewise greater.
Superior
nacre is more or less translucent, depending on its quality; and to the
iridescence of the outer laminae is added that of many interior ones,
so that the luster is vastly increased. The position of the pearl
within the shell may greatly affect the quality of the material and,
consequently, the orient. The choicest are commonly found within the
soft parts of the animal, and those of poorer quality are at the edges
of the mantle, or within the fibers of the adductor muscle of bivalves.
The
structure of pearls may be studied by examining thin cross sections
under the microscope, or by transmitted polarized light. It appears
that ordinarily a pearl is made up of many independent laminae
superimposed one upon another "like the layers of an onion," or,
