From either point of view, China is a well-deserved "pearl state". Our pearl history began 6,000 years ago in the Dayu era, according to the "Haishi Afterword" record, in 4000 BC, "three over the door without entering" dayu Ding "South Sea fish grass, pearl seal big shell" as a tribute.
In the twenty-four histories of the ocean, pearls are dazzling embellishments, and pearlescent treasures have become the best illustration of the status of royal aristocracy.
Figure 1|-point cui inlaid pearl stone golden dragon phoenix crown (Ming) (source network)
Let's set aside the glorious history of pearls for a moment and focus on the round, chubby gem itself. A central question: Why did pearls conquer all consumer classes at home and abroad in six thousand years? The answer is actually in the previous article: pearlescent. From ancient times to the present, the value and greatest beauty of pearls lies in its hazy and seductive luster.
Figure 2| various color pearls (source network)
So where does the most distinctive pearl in the world of this colored treasure come from? In the spirit of scientific truth-seeking (playful curiosity), Gigo and many partners in the laboratory made a bloody deposit, used a full set of equipment, and cut open N pearls, just to grasp the pearl. Without further ado, are you ready? Step into the world of pearls together.
What
What is acry?
Pearl is not a protein, it is the core factor in the radiance of pearls.
Nacre is one of the main factors that determine the quality of a pearl. Nacre is composed of nacre, more than 90 percent of which is aragonite (composed of calcium carbonate), followed by shell hard protein filled between the gaps in the aragonite stack.
Figure 3| Structure of the nacre (left: MAGmagnifying view, right: structure diagram)
The reason why pearls can present beautiful pearl luster and unique halo is because the light shines on the nacre, causing interference, diffraction and reflection, resulting in an optical phenomenon similar to the rainbow, hence the name "pearl luster".
Not only is there a pearl in the structure of the pearl, but also the mother oyster that produces the pearl has a pearl on it, so many mother-of-pearl oysters will also have pearl luster and halo. (Inadvertently, Ji Ge remembered oysters, that fat luster, tit-tut)
Fig. 4| the green and red color of Tahitian pearls, and the pearls on the black butterfly shells show a rainbow-like halo
How
Classification of pearls
So are all pearls made of pearls? NO、NO、NO。
According to the structure of pearls, pearls can be divided into two categories: nucleated pearls and seedless pearls. Most marine cultured pearls (except keshi beads) are nucleated pearls, all natural pearls and most freshwater pearls (except Edison pearls) are seedless.
Figure 5 | classification of the origin of pearls
Figure 6| Schematic diagram of the structure of different types of pearls
Natural pearls are formed by accidental conditions, when external impurities enter the shellfish, shellfish secrete pearls in order to protect themselves, and pearls are formed by wrapping foreign bodies layer by layer. (Shellfish wailed: Ah! hurt! ）
Both pitted and seedless cultured pearls mimic the formation process of natural pearls, except that nuclear cultured pearls place spherical bead nuclei into the mother of pearl, while seedless cultured pearls place the cell sheets of the mantle membrane into the mother of pearl. The mother shell is stimulated to wrap the pearl nucleus or the growth of the mantle membrane cells layer by layer, ranging from 1 year to 3-5 years before pearls can be formed.
Figure 7 | the formation of pearls
Why
How does the lab identify different types of pearls?
Pearls are not transparent enough to be visible to the naked eye, so the lab needs to identify the type of pearl with the help of X-Ray microscopic devices (yes, the X-ray you know). The X-ray microscopic device works the same way we went to the hospital to take X-rays.
Figure 8 | X-ray of shellfish
The three images below are X-ray microscopic images of natural, pitted, and seedless cultured pearls.
Tips: The following knowledge points, circled, to test
It can be seen that:
The inside of natural pearls is a concentric layer of growth structures from the inside to the outside, which are nacres composed of nacre. Sometimes there are radial structures near the center. Figure 9.
The structure of a nucleated cultured pearl is a uniformly complete round core on the inside and a concentric layer of nacre wrapped around the outside. Because pearl nuclei are usually ground from shells, the layered structure of shells sometimes occurs.
The structure of a seedless cultured pearl is a twisted small cavity (left behind by the absorption of small pieces of mantle membrane) and a concentric layer of nacre.
Fig. 9| Natural pearls (left) nuclear cultured pearls, the bead nuclei show a layered structure (middle) seedless cultured pearls (right) X-ray micrograph
How is the thickness of the bead layer measured for nuclear cultured pearls?
Cultured pearls, such as Akoya, Nanyang pearls, Tahitian pearls and Edison pearls, are composed of pearl nuclei and nacre, and the thickness of the nacre is called the thickness of the beads. Bead thickness measurement is a technical exercise!
GUILD Gem Labs experiments with a variety of bead thickness measurement methods, performing experimental analysis by cutting a large number of pearl samples. The "Gilder Bead Thickness Measurement Standard" was finally developed.
Figure 10 | X-Ray photomicrograph
1. How to measure the thickness of the nacre in the laboratory
To accurately measure the thickness of a pearl's bead layer, the laboratory needs to test the pearl with the help of an X-Ray microscopic device.
As mentioned above, as we do with X-rays, when X-rays pass through the pearl, leaving an uneven image on the detector, jewelry appraisers can judge the internal structure of the pearl and measure the exact thickness of the nacre by changing the brightness and grayscale of the image.
After X-ray microscopic phase assays on a large number of pearl samples of different kinds, the GUILD Gem Laboratory cuts and peels off the bead nuclei, using a per-cent caliper to measure the actual thickness of the bead layer. Through laboratory technical correction, the measured error range of bead thickness can be reduced to less than 2.0%.
At the same time, for each pearl sent to the GUID Gem Lab, we will select multiple test points in different directions of the pearl to consider it comprehensively, eliminating the error caused by the uneven distribution of the nacre layer.
Figure 11| Using calipers to measure the thickness of the stripped pearl nucleus nacre (left); X-ray measures the nacre thickness (right)
2. The influence of bead thickness on pearls
Whether the thickness of the bead layer can reach the standard range is an important factor in judging the quality of a cultured pearl with a nucleated culture.
First, the thickness of the bead layer affects the durability of the pearl. If the nacre layer is too thin, it will lose its luster if it cannot withstand the grinding of the years, and even the phenomenon of "breaking the skin" will leak out of the inner pearl core; secondly, the thickness of the nacre layer will also affect the luster of the pearl, the unique and soft luster of the pearl is mainly formed by light on the surface of the pearl layer, if the nacre layer is too thin, the pearl will also lose its luster.
Taking two Akoya pearls with different bead thicknesses as an example, it can be seen in the appearance chart that the pearl on the left is significantly stronger than the pearl on the right, both in luster and halo. Through X-ray detection, it was found that the thickness of the pearl layer on the left side was indeed significantly thicker than that of the pearl on the right, and the thickness of the pearl layer on the right side was calculated to be lower than the lower limit of the Akoya bead layer evaluation standard (measured 0.12mm), resulting in poor quality of the pearl.
Finally, the pearl was cut open for inspection, and the actual bead thickness data confirmed the X-ray test results. As shown in the figure:
Figure 12| Akoya comparison of different bead thicknesses
3. Different varieties of cultured pearls have different evaluation standards
The thickness of the bead layer of different species of cultured pearls varies according to their growth environment, formation time, and mother shell species. In general, Akoya pearls are generally in the range of 0.15-0.5mm due to their small bead size, short production cycle and thinner bead layer, while Tahiti, Nanyang white pearl and Nanyang gold beads are acceptable due to the large size of the beads, long production cycle and thick bead layer, and the thickness of the bead layer needs to reach 0.8-3mm.
The thickness of the pearl layer of each cultivar and the X-Ray image pair are as shown in the figure:
Figure 13| comparison of the thickness of pearl bead layers of different varieties
(From left to right: Akoya, Nanyang Gold Pearl, Tahitian Pearl, Freshwater Seedless Pearl, Nanyang White Pearl)
4. Description of the thickness of the nacre in the GUILD Pearl Grading Report
Figure 14 | The right side of the GUILD Pearl Grading Report lists the bead thickness (in mm, accurate to two decimal places).
The general range of bead thickness of the current pearl cultivar is described in the Moments on the left
Giga's Notes
"Rainbows shine through the curtains, the planets hold alleys, and treasures shine everywhere."
The earliest impression of pearls came from ancient poetry, yearning for that gorgeous and extreme beauty. Later, for the first time, I saw the pearls in series from the necks of my mother and grandmother, and I secretly decided in my heart that I must buy them the best and largest pearls in the future. Now that I have grown up and fulfilled the wishes and expectations of my childhood, I really like pearls, and the reason for my work is that I often have contact with this little cuteness.
The job of a gemologist is to shuttle between physics, chemistry, geography and other subject knowledge and a variety of large-scale identification instruments, but believe me, our love of gemstones is the reason for fighting for it all our lives. Only by in-depth study of various types of pearls and testing with different methods can effective results be obtained.
You see a reliable number on the certificate, a precise word, which may be the meaning of using X-Ray microscopic equipment, micrometers and other instruments to cut through countless pearls.
The beauty of pearls is moving, and it is a joy of life for more people to clearly realize where its beauty lies.
Article authors: Zhang Qidong, Lin Moqing