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Identifying pearls means analizing a logical sequence of interdependent
data. First of all, they are carefully observed with the naked eye in
natural and artificial light and then examined under the lens and
microscope. Afterwards, they are tested with ultraviolet and X-ray
fluorescence, X-radiography or xeroradiography and X-diffractrometry
methods and, if necessary, their density is calculated. Before beginning,
however, one has to have a clear idea of each pearl type and its
respective structure and distinctive features.
Natural pearls are made up of conchiolin and aragonite crystals
arranged in radial and concentric layers, with the central axis "c"
perpendicular to the layers. Therefore the pseudohexagonal bases are
always parallel to the spherical surface of the pearl.
Cultured pearls with nucleus consist of a round, mother-of-pearl
nucleus around which the layers of nacre (conchiolin and aragonite) start
to deposit in a concentric pattern. The fundamental concept that has to be
kept well in mind is that the aragonite crystals of the mother-of-pearl
nucleus have a different orientation from the aragonite crystals on the
outer layer of the nacre. In the mother-of-pearl, the aragonite is
arranged like so many bricks in flat and parallel layers, whereas in the
nacre the aragonite prisms have a concentric-radial configuration, with
the hexagonal bases always parallel to the spherical surface of the
pearls.
Cultured pearls without nucleus do not contain mother-of-pearl
beads, but are started by an organic nucleus composed of a fragment of
epithelium.
Imitation pearls do not contain nacre and their surface is
homogeneous and granular, often with scratches. Their interior can be made
of glass, mother-of-pearl or plastic.
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珍珠，光洁圆润，华光溢彩。传说中，珍珠是大海的泪珠，祖先们认为当一滴露珠落进 海里，正好被一只巨大的贝壳接着，就会生成一粒珍珠。事实上，天然珍珠是气泡或砂粒偶 尔进入贝壳，贝壳中的软体受刺激，不断排出珍珠液，一层层地包裹这一外来物而形成的。 这也可被哲学家解释为最美的东西往往是在痛苦的代价中磨练出来的。
熟悉了钻石的耀眼坚硬冰冷之后，2001年的时尚又青睐于珍珠的温婉雅致、含蓄纯美 了。今年春季的北京珠宝拍卖，少见钻石身影反而推出了数十件风格不同的珍珠首饰。（有 单粒的南洋白珍珠项坠、戒指，有塔希堤黑珍珠耳环、项链、别针底价万元以上，也有价值 几千元的淡水紫色珍珠别针，还有香港著名珠宝设计师罗 妍女士设计的淡水珍珠首饰）。 珍珠是最高贵的珠宝。清代的朝珠中，珍珠朝珠是只有皇帝、皇后方能佩戴的。英国皇 室中也有佩戴珍珠项链的传统。女王最喜欢佩戴的是2－3串珠的珍珠项链，配宝石别针。 在美国的珠宝拍卖会上，一串南洋珠项链，珠径13．5mm～17．8mm，以70万美元成交，但 珍珠又能为每一位女士所拥有。国内的珠宝收藏家可能会以20万人民币收藏一串直径10～ 13mm珍珠项链，白领可以一两万人民币买一串直径8～9mm的珍珠项链，年轻女孩可以几千 元买一件配钻石的单粒珍珠项坠。即使只花几十元，买一对小小的淡水养珠耳钉，也能为服 饰增色不少。
几十万的珍珠项链在国内市场还很少见，但也许有一天您会买一串，市场上常见到几千 元一串的珍珠项链。为什么有时看上去差不多的项链只卖几百元甚至几十元？几十块钱一对 的珍珠耳钉是真的吗？几十万美金一串的珍珠又贵在什么地方呢？
其实，判断珍珠的价值并不太难，掌握几点要诀就能看个大概齐。首先要看珍珠种类，天然珍珠现在除了拍卖会上有，别的地方几乎不可能见到了。所有你见到的真白珍珠都是养殖珍珠，大体上你会遇到南洋珠、日本珠、淡水养殖珠，此外还有复合珠、染色珠、假珍珠。
南洋珠的粒径在9mm～18mm，有白珍珠、黑珍珠、金色珍珠，南洋珠是海水养殖珍珠，养殖时间长，成活率低。但养成的珍珠光泽美，粒度大，价值也高，往往单粒珍珠镶成的首饰，可卖到几千至几万元。而其中黑珍珠更是近年来相当热卖的品种。
日本珠的粒径在2mm～9mm，也为海水养殖珍珠，粒径较小，国内较少见。
淡水养珠，粒径多小于9mm，形状圆的很少，是以套膜种于蚌中养成，养殖时间短，一个蚌中往往能收很多颗珍珠，产量大，所以价钱低。
复合珠，有马贝珠等，价值远低于同样大小的南洋珠。
染色珠，颜色较差的淡水珠会染成黑色、金黄色，市场上可见到粒径很小的成串淡水珍珠，染成黑色，冒充黑珍珠。
假珍珠，多用塑料制成。
另外，要看珍珠的大小、圆度、光泽度、瑕疵，以粒径大、浑圆光泽明亮、无瑕疵为上品。
Density
In itself the density of pearls is not a diagnosis for identifying the
typology. However, when added to the results obtained with the analyzes it
can sometimes have a decisive weight. In general, in natural saltwater
pearls the density is 2.68-2.70. Those with a not very crystalline centre
("blue" and "half blue") have lower density values,
ranging from 2.60 to 2.66. Freshwater natural pearls also have lower
values (2.65-2.67). The density of the pink pearl of the Strombus gigas,
more crystalline, varies from 2.81 to 2.87. Cultured pearls with nucleus,
which contain more mother-of-pearl (those from Mississippi had a density
of 2.82), have higher densities varying from 2.72 to 2.78. In the
freshwater cultured pearls without nucleus the average value of the
density is 2.67. Since pearls are organic material, heavy liquids should
not be used to find their density. It is thus necessary to use a
hydrostatic balance with distilled water, which is also much more precise.
With drilled pearls you have to eliminate the air in the hole with a
needle or thin drill bit before weighing them in water.
Tests with the lens
After having carefully observed the pearls with the naked eye to obtain
the first impressions of their color and sheen, a good aplanatic and
achromatic 10x magnifier is used. It is preferable to use an intense, cold
light as an illumination source, generated by optic fibers. First of all
the surface is examined, to see if the texture is like a "topographic
map", with sinuous and rippled lines or if it is uniform and
granulated. In the first case, the pearl is natural or cultured, in the
second case it is an imitation. The hole is then examined, if there is
one. In natural pearls you can note a continuous, layered structure, that
takes on a more intense or darker shade ("blue" pearls) towards
the center. In cultured pearls some exterior layers can be seen, a darker
(less crystalline) or pinkish (because of the added coloring) layer and a
homogeneous central zone. Both the edges of the hole have to be examined
to see if they coincide. In imitation pearls the edge of the hole often
does not have the thin, iridescent film that covers the surface. For
necklaces, to be certain about the diagnosis it is advisable to unstring
them and observe the pearls carefully one by one.
Tests with the microscope
The microscope serves to observe the features of the pearl surface in
greater detail. In strong light, with a 20 to 30 magnification, the
sinuous lines of natural and cultured pearls are easy to see and their
closeness can be calculated with a good approximation. In natural pearls
and in the cultured Japanese ones (Akoya), they are quite thin and close
together. In the cultured South Sea pearls they are more distant and more
marked. However this is only indicative and not diagnostic. The microscope
is much more useful for identifying treatments, especially those which
intensify the coloring. Cultured pearls with pink reflections often
present eosin residuals in the conchiolin layer situated between the nacre
and the nucleus.
The surface of dark pearls treated with silver nitrate often show a
non-homogeneous color with darker patches and violet or purplish zones.
Moreover, a certain quantity of black metallic silver, obtained when
ultraviolet sun rays and conchiolin reduce the silver nitrate, is visible
inside the hole. Irradiated pearls, almost always cultured, have a grey
nucleus, often with rings. This is because the radiations pass through the
outer layer of the nacre and darken the underlying nucleus. The color of
the pearl is given by the reflection of the nucleus. In the rare cases of
cultured pearls with assembled (shell dust and calcite bound by polymers)
and dyed nucleus, under the microscope the effect of the fusion of the
polymer, caused by the drilling, is evident inside the hole. For imitation
pearls, the microscope shows up the granular surface and confirms the
diagnosis.
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