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.
Thus the shell is being constantly enlarged at the edge, by a deposit
of the exudations of the mantle; conchiolin for the epidermis outside,
lime for the prisms and inner layers of transparent plates, until the
shell has attained its full growth in size, after which some varieties
continue to lay on nacre only.
[Illustration: MANILA PEARL-SHELL WITH THE LIP CONSERVED]
The linings of some have a black rim, extending from the hinge on one
side, around the edge to the hinge on the other side. Viewed from the
edge this dark band appears to be a sixteenth to half an inch wide
(widest at the lip), fading out as it becomes lost under the thicker
white nacre of the interior, but turn the shell up and look at it
squarely from the front and it is black only around the extreme edge
where it joins the epidermis. This kind of shell is found in the
Pacific about the islands of Polynesia and is called the black
shell. In others the nacre is white to the edge. The iridescence of the
white shell generally shows more play of color than that of the black.
The white shell is usually somewhat flatter and broader than the black,
and the epidermis is light yellowish-brown. This variety is found in
great abundance on the northern and western coasts of Australia. The
yellow, greenish and grayish shells (these colors refer to the edge of
the lining), are similar in every way, but inferior, the yellow being
the best of the three.
The shell lining of a common form of the unio, or fresh-water mussel
pictured at page 146, like that of the meleagrina, shows little
iridescence except at the edges outside the pallial lines, where the
nacre is comparatively thin, and at the striated surface of the scar
or bed of the adductor muscle. In quality of color and luster it is
inferior to the nacre of the sea fish, the white being more chalky in
appearance and the luster less pearly. The material of which the shell
is composed and its construction are however almost identical with that
of the salt-water mollusk. In fact all shells are made of the same
ingredients and are constructed on the same general principles by the
animals inhabiting them.
[Illustration: MISSISSIPPI NIGGER-HEAD PEARL MUSSEL]
This description of pearl shells has been given here because a
knowledge of the shell enables one to understand the formation and
characteristics of a true pearl, and the differences which exist
between the gem and other similar formations formed in pearl and other
oysters, mussels, and univalves. Many such formations are found, having
the elements and constructed like one or both of the outer parts of
the shell, and some, in part like the lining, but these are not true
pearls; the gem has neither the material nor construction of the middle
and outer shell. Except that the pearl, because of its form, is rarely
iridescent even to a slight degree, whereas the nacreous lining of some
pearl-bearing shells is brilliantly so, the pearl and the nacre of the
shell in which it grows, are essentially the same. Pearls are more or
less spherical and independent formations, made by the fish on the
same plan and from the same secretions with which it lines the shell,
misdirected by abnormal conditions. Those constructed like any other
part of the shell are not true pearls.
The normal instinctive action of the mollusk is self-protective and
adaptive. By the secretive action of its mantle it gathers from the
water in which it lives, material to build a shell with a rough and
rugged exterior for its enemies, and adapted to resist the chemical
activities by which it is surrounded, and a perfectly smooth lining
suitable as an interposition for its own delicate organism.
Barring accidents, the building functions of the animal are employed
only in the extension of the shell to meet the needs of its own growth
and protection. But should a particle of secretion intended for the
shell, harden within the folds of the oyster's mantle, or some parasite
or other intruder present itself within the nacre-forming sphere, the
instinctive action which lines the rougher part of the shell is also
directed toward the foreigner, and it is at once covered with a like
deposit. This is the birth of a pearl, and it grows layer by layer as
long as it remains within the scope of the nacre building instinct.
These layers, or skins as they are called, are seldom iridescent.
Occasionally a pearl of that character is found, but it is generally
from a fresh-water mussel, and the nacreous plates are of unusual
tenuity.
Although the pearl like the lining of the mollusk's shell is composed
of carbonate of lime in series of thin waves lapping each other, each
series constituting a plate or separable layer, there is a distinct
difference in construction.
Whereas the lining is a series of horizontal layers, the pearl is made
up of concentric layers, each addition enveloping those preceding
it. These skins however are not always absolutely distinct and
separate. Instead of being like a succession of globular skins, each
completely covered by its successor, the growth is often spiral and
the construction is as if the nucleus had been rolled one, two, or
three complete revolutions in a continuous plate of nacre, and the
spiral envelope then finally merged into another plate and the process
repeated. That which to a casual glance, therefore, appears to be six
rings of nacre in a sectional cut, is in reality, several spirals of
two or three turns each.
It is also noticeable that whereas the wave edges, with all their
eccentricities, trend generally in one direction in the shell nacre,
in the pearl, the lines twist and curl with a concentric tendency, as
though the waves had been laid on by turning or rolling the pearl in
the material of which it is composed.
A white pearl on being cut in half shows a number of faint dark rings
one within the other, from the surface to the nucleus in the centre;
usually these rings occur at almost regular intervals. Upon close
examination under the microscope, it will be seen that the inner part
of these intervals is white, and that the color gradually changes to a
yellowish tint which deepens until it culminates in that which appears
as a dark line against the succeeding outer formation, the material of
which is also white in the beginning. Although this change of color is
very slight, a section between two rings will often show three distinct
bands; the inner white, the centre one faintly yellow and the outer
one of a deeper tint. In some cases the dark concentric rings succeed
each other very closely, in which case no abrupt changes of color
between them are noticeable. The material occupying the space between
the rings is the sectional appearance of the skin of pearl. Upon
applying a weak acid to the surface of an entire section of a pearl,
it effervesces, and the inner colorless parts of the bands are at once
attacked. After several hours the white inner part of the skins will
show depressions where the calcium carbonate has been dissolved, and
the outer parts of the skins will be marked by coarse black rings of
undissolved animal tissue, similar in appearance to the epidermis of
the shell. Now as these skins are made up of many very thin waves of
calcium carbonate lapping each other and set in animal tissue, it would
appear, therefore, that in the beginning these waves of transparent
calcium carbonate are set in animal tissue of extreme tenuity and that
the proportion of animal tissue increases with the growth of the skin
until it reaches a stage provocative of a new skin, which begins with
purer layers of the smoother crystallized mineral like its predecessor,
and identical with the nacre of the shell. If this be so, it would
account for the various tints of color and degrees of luster in white
pearls and for the fact that the outer skins of very lustrous pearls
are usually very thin also. Similar conditions exist in colored pearls,
though the presence of a pigment makes them less noticeable. The skins
of the haliotis pearl, which separate easily, usually show remarkable
luster on the inner surface.
Sometimes the nucleus is surrounded by a confused mass without apparent
concentric markings, as though it had been enveloped in nacre which had
solidified while stationary, or the first deposit shows the concentric
skin arrangement at one segment of the circle only; followed by layers
which appear in the depressions of the mass and are continued until
they finally include the whole pearl. These layers are usually very
thin, and the partial or segmentary layer formation is quite common in
the early stages of the pearl's growth. At that period the concentric
lines are also irregular, and in many cases where the curve is true,
they extend about one quarter of the circumference only, another
concentric skin being lapped on the ends, as though the globular skin
had been formed in sections.
As before stated, it often happens that the skin division lines are
spiral, as though the nucleus had been rolled one way in the nacreous
material. In all cases the first deposits of a skin, that is the first
of the nacreous waves of which a skin is composed, appear to be most
transparent and lustrous. The component waves of nacre then gradually
become more impregnated with animal tissue until they apparently reach
a stage which induces either a rest on the part of the fish, to gather
nacreous material, or a new deposit of less impure nacre, to protect
itself from the increasing impurity of the pearl's skin.
The skins undoubtedly mark certain stages in the formation of
the pearl, though the skin and the nacreous waves of which it is
composed are often confounded. In the skinning of pearls an entire
skin is seldom peeled off. The surface is scraped, a number of the
component waves being taken off, until the luster is improved and
it is then supposed that the entire outer skin has been removed. A
close examination however, will show, by breakages in the surface
of the waves, that the under skin with its peculiar and systematic
arrangement of surface wave edges, has not been reached.
A sectional view as seen in a half pearl would lead one to infer that a
free pearl in the beginning lies stationary in the oyster; is turned or
partially rolled as it grows larger; and finally, on attaining about a
one grain size, is kept in constant motion with a concentric rolling in
the nacreous exudations of the mantle which are deposited upon it.
The nuclei of pearls were long thought to be grains of sand, but late
and careful research has shown that in the majority of cases they are
minute parasitic or domiciliary worms.
Professor Herdman and James Hornell, after three consecutive
inspections of the oyster banks in the Gulf of Manaar in 1902-3, stated
in a paper contributed to the British Association for the advancement
of science, that after examining many hundreds of oysters and
decalcifying a large number of pearls, they had come to the conclusion,
that grains of sand and other inorganic particles formed the nuclei of
pearls only under exceptional circumstances, as for instance, when the
shell was injured by the breaking of the ears, which would enable sand
to get into the interior.
Pearls, or pearly excrescences on the interior of the shell, were due
to the intrusion of leucodore, clione and other borers. Pearls found
in the mussels, especially at the levator and pallial insertions, were
formed around calcospherules, minute calcareous concretions produced
in the tissues. But most of the fine pearls found free in the body of
the Ceylon oyster, contained the remains of platyhelminthian parasites.
These observations agree with the opinions formed, after careful study,
by several eminent conchologists.
The action of the mollusk results differently as the object to
be covered is free within the folds of the creature's mantle or,
rising above the surface of the nacreous lining, presses upon it. If
free, the intruder is enveloped by the animal's exudations and the
deposits become concentric instead of level, or nearly so, as in the
construction of the shell. It is said that the foreign substance acts
as an irritant, causing the fish to exude its secretions abnormally in
order to protect itself, and thereby creating a diseased condition;
but from the fact that the process continues after the intruder has
been enveloped and rendered as non-irritant as the natural lining of
the shell, it would appear that the introduction of a foreign element
simply draws upon it the normal impulse of the fish to cover with nacre
anything with which it comes in contact, and that the method of doing
it is similar to the instinctive rolling action of the tongue when some
insoluble globule is put in the mouth, for not only do free pearls grow
spherically, but a nucleus fast to the shell is not covered simply but
it grows to a pearl, round and domelike, as nearly spherical as its
juncture with the shell will permit.
Not only is the composition of a pearl identical with the lining of
the shell where it is formed, but in a general way its appearance and
characteristics are the same, except that free pearls are sometimes
colored when the nacre of the shell is white.
Button pearls, warts and baroques, grown fast to the shell, are usually
like the surrounding nacre in every respect.
Salt-water pearls are characterized by the soft velvety luster of the
oriental mother-of-pearl, and fresh-waters, like the lining of the
unio, have a somewhat thinner looking and more chalky texture.
Abalone pearls have the irregular surface and coloring of the haliotis.
Conch pearls resemble the delicate pink china-like lining of the shell,
and clam pearls have the glazed earthenware appearance of the inside
of a clam shell. The one material difference between a pearl and the
lining of the shell in which it grows is, that in the one case the
fish deposits the nacre over an even surface, and in the other wraps
it around a central point with delicate precision in successive filmy
layers.
Dissection shows that a pearl during growth is liable to many mishaps.
As with the human creature, a promising youth may end in a wretched
maturity. It is also possible that an ugly period may be redeemed
by later happenings, and the thing that was worthless in its early
existence, be found in its age worthy of a place among the great gems.
Pearls found with a dull, chalky exterior sometimes have lustrous skins
beneath. Sometimes a bony-looking formation will be found, on breaking
it, to have a variety of skins in the interior, some of which are very
lustrous, others white and chalky, like the middle shell of the mollusk.
Many of these dead pearls are formed throughout of this material.
Others, perfectly spherical, are simply successive layers of prism
groups like the conchiolin plates of the shell. Upon cutting these
through the centre the skins are shown by the concentric rings marking
their divisions and the prismatic formation appears as glistening
lines radiating from the nucleus to the surface. Under the microscope
these layers, which are thicker than the nacreous skins of true
pearls, appear identical with the epidermis plates, except that they
are concentric instead of flat, and are free from the coarse, rough,
conchiolin deposit which forms the extreme outer coating of the shells.
This deposit is also found, however, in some pearl formations, as
many of the abalone baroques, especially when they are somewhat flat
in shape, are like two pearl blisters joined, with the shell-building
process reversed, the rough, black conchiolin being inside, and the
nacre outside. Undoubtedly pearls containing hidden qualities which
made them once gems are thrown away as valueless, while others found
just as nature had covered their earlier coarseness with a coat of
beauty, are worn and excite much admiration for their skin-deep beauty.
Though the successive skins of a pearl do not usually vary much in
color, except in abalone pearls, it does happen occasionally, for the
removal of dark yellow skins sometimes discloses another of better
color—a good pink for instance. From the sectional appearance of pearls
it seems probable, that in the majority of cases the color of yellow
pearls would be improved by the removal of the outer waves of the outer
skin.
Changes in shape sometimes occur during the growth of the pearl, the
tendency being always toward the rounding of the surface. If the
nucleus is fast to the shell, a dome is built over and around it. If
the nucleus permits, the nacre is deposited not only over but under
its edges to the point of contact with the shell, so that a button
pearl connected with the shell at the centre only, results. Two
pearls held against the shell and growing side by side are separately
enveloped until they touch each other, after which they are included
in single deposits of nacre and the depression between their domes
becomes less distinct with each successive coating. Similarly, a
cluster of small pearls lying together often forms the nucleus of a
large rounded baroque or button pearl. Examination of such formations
shows, that up to a certain period the pearls have a separate existence
and growth. They then become joined in an irregular mass of twinned
pearls, and finally, if allowed to remain in the oyster long enough,
all individuality is lost in the tendency to round over. The same
thing occurs when grains of sand or other intrusions become attached
to a growing pearl. They are quite prominent when first included
in the nacreous deposit and can be easily detached from the under
pearl by breaking through the layer which binds them on; but they
are soon obliterated by succeeding deposits. This filling-in process
is sometimes accomplished by additional layers in the depression,
sometimes by thicker layers. It happens occasionally, when skinning a
round pearl, that one of these fillings is uncovered and flakes out,
leaving the pearl irregular, as it was in a former stage of its growth.
Although pearls naturally grow spherically, many free pearls are more
or less buttoned, that is, have a flat place from which the pearl
rises like a dome, high or low. This happens when the pearl is held
during growth by the fish against the shell with a part of its body
intervening. According to circumstances, the pearl varies in form from
slightly button, to a low dome, rising from a plane at its greatest
diameter. Should a pearl of this description become dislodged, the
rounding action of the mollusk would begin at once to obliterate the
plane.