of the pearl from the mollusk during a transitional stage, and that
the presence of spots and streaks of that character, scattered over
an otherwise lustrous surface, indicates that the secretions of the
creature's mantle did not hold some essential ingredient in sufficient
quantity to secure perfect crystallization and thereby cover the entire
surface with transparent plates of calcium carbonate. It may be also
that a lack of essential elements in the creature's exudations, causes
a cessation of the mantle's action which by all signs appears necessary
for the production of transparent plates of nacre.
"Peelers" are pearls of imperfect skins having indications of a better
one underneath. Speculators buy these pearls at a low price and skin
them. Sometimes they are rewarded by a smaller, but much more valuable
pearl. Many times the under skins are no better or worse, or if better,
the loss in size and weight, together with the cost of the work, make
it unprofitable.
Peeling should not be attempted with cylindrical shaped pearls having
chalky bands or rings around them, as such imperfections usually
penetrate to the interior in pearls of that character. Cylindrical
pearls are almost invariably fresh-waters. The imperfections disclosed
in the under skins by peeling, are commonly irregularities of shape
which have been rounded over to the improvement of the sphericity of
the pearl.
It is currently reported among the pearl hunters who fish the
western and southern streams, that the finding of soft pearls is not
infrequent. Upon opening the mussel, they sometimes see through the
mantle of the creature, an apparently fine pearl which upon being taken
out proves to be a soft jelly-like substance, the form of which is
usually destroyed in squeezing it out. These men do not believe that
a pearl is formed in layers, but think that all pearls are originally
globules of a similar soft substance, hardening later to a compact
solid ball and they call them "mussel eggs."
Many pearls taken from the small thin-shelled varieties of the ocean
mollusk, as for instance those of Venezuela, are devoid in part, or
wholly, of the nacreous luster and instead have a china-like or waxy
luster, or a dead chalky skin. A large proportion of the Abalone
pearls and baroques are lustrous only in part, one section having an
earthenware appearance. Many appear to be formed of interstratified
layers of nacre and conchiolin. This construction is very distinct in
a formation peculiar to the Abalone, consisting of two nacreous shells
joined perfectly at the edges, the inside walls of both being covered
with rough black conchiolin.
Peculiarities in the quality of the nacre sometimes give an appearance
of uneven shape which does not exist in reality. The light falling
upon such pearls produces a knobby effect, as though there were
protuberances on the surface. The texture of others is such that when
looked at squarely from the front they appear pyramidal in form, the
rounded apex pointing toward the observer. Such pearls have a soft,
waxy appearance generally.
Another common imperfection consists of pits in the surface. These may
result from various causes: in many cases from the dislodgement and
rolling of a pearl which has been flattened during earlier stages by
pressure in one position against the shell. Freed from this hindrance
to spherical growth, the later concentric layers would round over
the edge of the flat spot and thereby leave a pit, or cavity, in the
centre.
In other cases pressure against the pearl, or the partial inclusion
of foreign substances, especially of an organic nature which decay
before being entirely covered, are possible causes. The reverse of
this also occurs; grains of sand or other minute particles adhering to
the surface are covered by succeeding layers, thereby producing knobs,
more or less observable according to the lapse of time between their
inclusion and the taking of the pearl from the oyster.
If undisturbed, the fish will by the deposit of sufficient layers of
nacre, fill the intervals and round the surface again. That this is
done in time is shown by the occurrence of pearls having an even dome
over a nucleus formed by a cluster of small round and irregular pearls
enveloped together. In the process of skinning, or the removal of one
or more of the layers of nacre, it is sometimes found that a depression
has been filled by a thickening of the deposits in the hollow; at
other times extra layers fill the space, and these flaking out with
the outer skin reveal the hidden irregularity which lay beneath the
round surface, thus necessitating the removal of several entire skins
before a sphere is reached again. The under skins of some pearls appear
to have failed to completely envelop the nucleus. The cavity resulting
is then filled to an even surface and is succeeded by fully developed
skins. It is, therefore, not certain that a pearl, perfect in form and
skin when found, has been so at all stages of its growth. Broken pearls
sometimes show not only differences of color but of thickness in the
successive layers. The skins of fresh-water pearls especially are often
very irregular in thickness.
Many pearls have cracks in them. These generally escape the observation
of inexpert persons, as they are usually under the outer layer.
The fact that they rarely extend to the surface suggests that the
solidification, or drying out of the confined interior layers, may be
the cause. These are considered detrimental and dangerous by dealers,
so that pearls with cracks in them will not bring as high a price as
they would if free from them.
As cracked pearls are liable to break, especially when pierced for
stringing, it is well to avoid them, though the percentage of those
which do break is small. In reality these cracks are more of an
imperfection than a danger. Occasionally they are quite noticeable and
are then a bad imperfection, but frequently a sharp eye or the loup
only will detect them. Surface cracks however are quite perceptible.
They are dangerous and are considered a serious imperfection.
There is a peculiarity of rare occurrence which, as it is a departure
from the ideal, may be termed an imperfection, though some regard it of
value as unique. It is a similarity under the surface of some pearls
to a metal which has been hammered into small flat spots identical in
appearance with the jewelry in vogue during the latter part of the 19th
century made of "hammered gold." It is scarcely noticeable except under
a loup, when the fine lines dividing the confused planes appear. These
pearls are usually slightly pink or pinkish yellow. Sometimes these
planes resemble the facets on a cut diamond, generally lozenge shape,
and often grouped similar to those on the under side of a diamond.
Small holes and blisters on the surface are quite common, but
ordinarily they are scarcely perceptible to the naked eye.
Many faults can be concealed by the jeweller when the pearl is mounted.
Slightly buttoned pearls are set on a peg in the centre of a small
shallow cup; they then appear quite round. A spot, blister, or cavity,
in a round pearl can be obliterated by pegging, or hidden in the
setting. Great irregularities in the sphericity are lost to the eye
when the gem is set in the prongs of a ring or other piece of jewelry.
Pearls shaped like a double convex lens may be made to look round,
or very nearly so, by piercing them so that the flattened domes are
brought in contact on the cord holding them together as a necklace.
Piercing and stringing obliterates or hides many flaws. By careful
selection, the jeweller can utilize pearls having a blemish by drilling
through the spot where the flaw is, and if there is another on the
opposite side that also will disappear. Other imperfections near the
hole are often hidden in necklaces, as they cannot be seen when the
pearls are held close together on the string. It is for this reason
that a string of pearls can often be bought for less than a like number
of loose pearls apparently no better but which in reality are much
more perfect in shape and free from flaws. Imperfections unseen in the
strung pearls would be quite noticeable in the loose and undrilled.
The irregularities of baroques cannot properly be called imperfections;
nevertheless a baroque is more valuable as it is free from indentations
and approaches the round in appearance, or has sides which will give it
a round face when mounted. The curious forms into which nature moulds
many of them are very attractive, and as they lend themselves to the
imaginative skill of the jeweller, are valuable. The faults common to
them are rough places uncovered by nacre and colored streaks or spots,
usually yellow tending to brown. These discolorations are confined
generally to the point where the baroque was attached to the shell, but
not infrequently they extend far enough to leave no front which would
be quite clean to the eye, when mounted.
Oriental baroques as a rule are more lustrous, more even in shape and
seldom discolored. Many of them are sufficiently regular to string for
necklaces, and some can be used in jewelry so that on the face they
appear like round, drop, or pear-shaped pearls.
GENESIS OF PEARLS
[Illustration: TUAMOTU PEARL-SHELL]
Pearls are found in certain marine and fresh-water mollusks. The
former are usually termed oysters, though zoölogists regard it in some
instances as a misnomer. The sea-fish is the avicula margaritifera, a
bivalve of which there are many varieties, all of similar shape and
nature but differing widely in the size, weight, coloring, and quality
of the shell.
Of them, the genus "meleagrina" is the largest, has the heaviest shell,
and furnishes the greatest quantity of the beautiful substance known as
mother-of-pearl. The other extreme is the small, frail-shelled variety
taken off the coast of Venezuela, called sometimes avicula squamulosa.
Similar to this is the margaritifera vulgaris, or avicula fucata, of
Ceylon. The pearl oyster of the Persian Gulf though similar is somewhat
larger.
Exact and uniform classification of the pearl-bearing mollusks of the
sea does not exist, nor is it necessary in this connection, as the one
distinctive feature which places them in the class under consideration
is the possession of a nacreous lining to the shell, for no shell fish
can produce a true pearl without it. The fresh-water pearl-bearing
mollusk is a mussel, unio margaritifera, also found in many varieties,
but all characterized alike by the nacreous lining of the shell.
These creatures, living upon the earth where water always covers
it, create in the building of their habitations a material of great
beauty, and sometimes produce gems which princes covet. Of the most
delicate nature, they build for themselves out of the water by which
they are surrounded, houses strong and enduring, fitted for their
protection from the rough chances of life, yet so furnished within that
they suffer no inconvenience from the rugged strength which encloses
them. Few things are coarser than the exterior of these domiciles,
but nothing in nature is finer or more exquisitely beautiful than the
substance with which they are lined.
The avicula margaritifera is a habitant of the coral reefs and shoals
about the islands and shores of the tropics; there are none living
now in northern latitudes, though fossils of many species are found
north of the present boundary of their habitations. An idea can be
formed of the general shape and appearance of pearl-oyster shells by
the neighboring illustrations of three varieties. These show the two
extremes of the marine mollusk, the meleagrina of the South Sea and
Australia, and the squamulosa of Venezuela.
[Illustration: AUSTRALIAN PEARL-SHELL]
In some of the small species, that of the Venezuelan Coast for
instance, the outer shell is yellowish, with fan-like markings of dark
reddish brown radiating from the boss or beak and growing darker as
they near the lip. This shell is thin and frail. The nacreous lining is
also thin but brilliantly iridescent and shows a series of fine lines
and irregular fissure-like markings extending outward from the hinge
and crossed by bands of color which curve with the outline of the lip
edge of the shell.
These colors, as brilliant but more evasive than the hues of the
rainbow, are not due to the presence of a pigment; they arise from a
phenomenon of light and form one of the most wonderful illustrations
of the ease with which our senses play tricks upon judgment and
understanding. It is the striated surface and the very thin transparent
plates of nacre, which cause a double interference and produce the
beautiful iridescence peculiar to the lining of these shells.
"Interference," as it is called, is an optical phenomenon arising from
two causes. When light falls upon a sufficiently thin transparent
surface covering a denser substratum not exactly parallel with it,
part of the light is at once reflected. Of that which passes through
to the under surface a part also is in turn reflected through the
first surface, and the confusion of rays or "interference" resulting,
produces to the eye the sensation of color.
[Illustration: VENEZUELAN PEARL-SHELL, WITH PEARL ATTACHED]
A familiar illustration is seen when a thin film of oil is spread over
water. The other way in which iridescence by interference is produced
in shells, may be demonstrated by drawing fine lines close together on
glass with a diamond. Light falling upon them will make the surface
iridescent. Melted wax dropped upon this striated surface would, upon
removal, show a like iridescence, reproduced with the impression of
the fine lines. The outer markings of the large Australian shell are
similar to the small Venezuelan. The mother-of-pearl interior is not so
iridescent.
Pearls and the shells in which they grow are composed almost entirely
of calcium carbonate or lime. A small percentage of organic matter and
water are the other ingredients.
As pearls are accidental and the result of a misdirection of normal
processes, a general knowledge of those processes is necessary to an
insight into the nature and genesis of the pearl, and as pearl shells
and the pearls in them are constructed on the same general plan, a
knowledge of the former will assist to a better understanding of the
gem and its eccentricities. The mother-of-pearl shell is built up of
a series of calcium carbonate plates or prisms set in organic matter.
In the material of the inner shell, the calcium carbonate greatly
preponderates; on the outside of the shell, the organic matter is
largely in excess. In the building of its shell, the animal deposits
the finest material and does the best and most compact work where
the house is in touch with itself, the walls becoming coarser in
construction and quality as they approach the outer surface.
In the inside of the shell, the calcium carbonate plates are very
fine and transparent, and the animal membrane in which they are set
is of extreme tenuity. In the middle shell these plates become more
chalky and less compact; in the exterior shell they are set in a
thicker binding of organic matter and terminate outside in rough, horny
fringes, completely covering the shell.
In a general way therefore, the animal deposits the best of its
secretions about itself and pushes out to the outer extremities, the
coarser elements which are fitted to preserve the finer parts of the
shell, as the finer parts of the shell are fitted to protect the
delicate organism which they enclose. The building of the shell is done
by a membraneous covering of the fish which entirely envelops the body
and is attached to the shell a short distance from the inner edge,
leaving a rim of membrane free around the fish and the edges of the two
valves. This membrane is called the mantle. It extracts lime from the
water, and at different parts exudes modified solutions of it mixed
with animal tissue, suitable for the construction of the various parts
of the shell.
The exterior of the shell or epidermis consists of conchiolin,
an organic compound. It is a horny-looking substance, and in the
large salt-water shells and in most of the fresh-water mussels, the
nigger-head of the Mississippi Valley especially, it appears to the
eye as a series of extensions, sometimes terminating in ridges,
which curve about the umbo and spread to the edge of the shell, each
extension coming from under the one preceding. In some varieties it
is attached as an excrescence to the prismatic formation immediately
under it, and may be easily detached in thin flakes: a rusty black in
some, brownish-yellow in all on the inner surface and in some on the
outside. The substance is generally opaque, but contains spots of which
some are translucent, resembling horn or amber, while others are more
transparent, similar in formation to the inner parts of the shell.
In most of the marine and fresh-water varieties, unlike the
nigger-head, the conchiolin exterior does not easily flake off. In
these the outer shell is composed of wave-like plate extensions,
superimposed one upon the other recedingly from the lip to the umbo
as in the others, but without the ridges, the plates being flat and
the edges more irregular. These extensions are formed of a number
of horizontal composite plates, which penetrate the shell to the
mother-of-pearl.
Not only may they be separated into thinner horizontal plates, but
they divide vertically into prisms. Under the microscope the edge of
a composite plate appears as a number of prisms placed side by side
lengthwise across the plate edge, but showing dark, intersecting lines
through the series where they divide as plates.
These prisms appear on the face of the plates as translucent hexagons,
separated by dark lines like a tessellated floor, and under a powerful
microscope are seen to be composed of similar smaller particles,
also joined together by a binder of tissue. The exposed parts of the
epidermis plates, forming the outer skin of the shell, are more dense
than the unexposed portions; the hexagonal dividing lines are thick and
blurred, and the faces are almost opaque, whereas in the unexposed
parts, the faces are translucent and the hexagonal markings are clear
and fine.
Though constructed in the same way throughout, these plates appear to
follow the general plan of shell construction, the preponderance of
calcium carbonate in the interior parts gradually changing to an excess
of organic matter as they become exposed to form the outer part of
the shell. The outer shell is in some varieties of a brownish-yellow
with radiating fan-like markings of a deeper tint or red; in others,
dark gray and brown to almost black. Immediately under the surface,
the plates become lighter in color, and finally almost white as they
approach the nacreous interior.
In all varieties the outer plates lie almost parallel with the
extension of the shell, so that, lapping each other as they do, the
outer contour of the shell is raised by a series of low steps from
the edge to the umbo. These plates appear to have been superimposed
one upon the other. On the contrary, they are added on the under
side. Starting from the umbo, which is the oldest part, the shell is
enlarged by the addition of a succession of plates from beneath, each
series extending a little beyond its predecessor, the rough conchiolin
fringe at their extremities forming the outer covering of the shell.
Following the growth of the epidermis, the shell and the lining are
also extended and built up, so that the entire shell is constantly
pushed to dimensions necessary for the proper and commodious housing of
its growing tenant.
Under the thin coat of epidermis on the Unio nigger-head, is a stratum
of prism plates similar to the outer plates of the Venezuelan oyster.
The prism faces are however smaller and the organic intersections are
thicker and darker. Immediately under and abutting, is another series
of plates which penetrate the shell almost horizontally at the lip end,
to the lining; diagonally at the thick part of the shell near the umbo
to another series of the same kind. Here, owing to their diagonal set,
upon peeling off the epidermis and the epidermis plates, the edges
appear as a series of fine lines curving about and spreading out from
the umbo. The plates set outward, away from the umbo, from the lower or
inner edge.
The effect is similar to that made by a pack of cards set diagonally so
as to spread the edges sufficiently to show the merest trifle of the
faces of the cards between the edges. The arrangement of these plates,
not only produces a series of fine lines curving about the umbo, but,
as the edges are slightly irregular, another series of fine lines
cross the others at right angles, radiating from the umbo. This doubly
striated surface, by interference, produces an iridescence more full of
color than the mother-of-pearl of any but the thin-shelled varieties.
Though similar in construction, these plates differ from those of the
epidermis. In some respects they suggest a transitional stage between
the outer and inner shell. A plate, as it separates from the series and
which appears as one line in the striated surface of plate edges, is
in reality a number of very thin plates, or waves, so welded together
that they cannot easily be separated. In this and the presence of fine
surface lines marking the wave edges, they resemble the nacreous plates.
The composite plate is opaque, but when split so that light can
penetrate there appears on the face, markings similar to the unexposed
portions of the Venezuelan epidermis plates only the hexagonal faces
are very much smaller and less distinct. So also the edge of the
composite plate appears as series of prisms crossing it from face
to face on the plate, in sets which show plainly, lines marking the
juncture of the individual plates or waves. Although the individual
plates or waves, can only be separated with great difficulty, together,
as composite plates, they can be flaked off from the shell very easily,
and they crumble and break into fragments under slight pressure.
The component plates or waves are very thin, and appear under the
microscope as white and translucent planes marked by outlines of the
prism faces.
The inner series of these plates as they near the nacreous lining
become harder and more compact, and incline more and more to a
horizontal position, so that at the point where they abut upon the
nacre it is not easy to distinguish them from the nacreous plates. At
the thinner end of the shell, about the edges, the plates are all of
this nature. They grow more friable and chalky as they incline to the
perpendicular, where the series are more numerous and are situated at
the thicker part of the shell about the umbo.
Adjoining the inner edges of the middle shell plates is the nacreous
lining. In this the calcium carbonate takes the same form as the
mineral aragonite and is identical with it. As a mass however, the
specific gravity is somewhat less, owing to the inclusion of organic
matter with the mineral in the shell. This material is harder, finer,
more compact, and contains less organic matter than that of which the
middle and outer shell is composed.