rather,
resembling the leaves near the upper part of a well formed cabbage.
When subjected to sufficient heat, the laminae separate from each
other, as do shells of edible oysters and similar mollusks under like
conditions. When broken by a hammer, a pearl may exhibit this
laminated formation. If not split directly through the center, the
central section may retain the spherical form ; and as this commonly
remains attached to one of the parts, its concave impression appears in
the other portion of the broken pearl. The outer laminae of many pearls
may be removed with a fair prospect of finding a good subjacent
surface, and this may be continued until the size is greatly reduced.
These laminae are not always similar in color or luster.
However,
not all pearls are laminated in this manner. Instead of superimposed
layers, some of them exhibit a crystalline form, composed of beautiful
prismatic crystals radiating from the center to the circumference. In
at least one oriental pearl examined, these crystals were in well
defined arcs, and were further separated into concentric rings of
different degrees of thickness, depth of color, and distance apart.
Another specimen—a Scotch pearl—combined in separate layers both the
laminated form and the crystalline structure.
Dr.
Harley points out that some crystalline pearls apparently originate in
mere coalescences of mineral particles, rather than in well defined
nuclei.1 Microscopic sections of crystalline pearls convey
the idea that the prisms branch and interlace with one another, and
also that in some instances they are of fusiform shape. However, these
appearances seem to be due simply to the cross sections having cut the
prisms at different angles.
Pearls
showing these types were exhibited at a meeting of the Royal Society of
London, June 8, 1887. That exhibit also contained a section of a west
Australian pearl of curiously complex crystalline formation; instead of
one central starting-point, it had more than a dozen scattered about,
from which the crystalline prisms radiated in all directions.
Since the three superimposed layers of the shell are secreted by separate parts of the mantle, viz., the
nacre by the general surface, the prismatic layer by the inner edge,
and the epidermis by the outer edge, it follows that if a pearl in
course of formation is moved from one of these distinctive portions of
the palial organ to another, the nature of its laminae changes. Thus,
if a pearl formed on the broad surface of the mantle is moved in some
way to the inner edge of that organ, it may be covered with a prismatic
layer; if then moved to the outer edge it may receive a lamina of
epidermis, and then by changing again to the broad surface of the
mantle it receives further coats of nacre.
1 Harley, "Proceedings of the Royal Society of London," Vol. XLV, p. 612.
