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Pietre prețioase organice: caractere de perlă, jet, amonit, MOP, chihlimbar și altele
Gemological basics like color, transparency, Luminescence, luster and others
Organic gemstones are one of the materials that humans can most easily obtain from nature, from bone ornaments in primitive society to shell currency in the Shang Dynasty, from rhinoceros cups in the Tang Dynasty to ivory Hu in the Ming Dynasty, and from helmeted hornbills in the Qing Dynasty to modern conch pearls. These organic gemstones are universally regarded as precious natural gifts, symbolizing identity and wealth.
Tabla de conținut
Section I The Concept and Common Varieties of Organic Gemstones
1. The Concept of Organic Gemstones
Organic gemstones are formed from ancient and modern biological processes. Organic minerals or gemstones that meet gem processing requirements are derived from animals, plants, and microorganisms. Natural organic gemstones have a warm color and enchanting luster (Figures 4-1-1, Figure 4-1-2).
Figure 4-1-1 Pearl
Figure 4-1-2 Amber carving (the spherical one is a pearl)
2. Common Varieties of Organic Gemstones
Common items in the market include pearls, Amber (Figure 4-1-3, Figure 4-14), ivory (Figure 4-1-5, Figure 4-1-6), etc. Others include fossil ivory(Figure 4-1-7), tortoiseshell(Figure 4-1-8), coral (Figure 4-1-9), jet (Figure 4-1-10), antelope horns (Figure 4-1-11), ammonite (Figure 4-1-12), abalone shells (Figure 4-1-13), and tridacna (Figure 4-1-14), melo pearls (Figure 4-1-15), and conch pearls (Figure 4-1-16). Cultured pearls (referred to as “pearls”) have some artificial factors, but the cultivation process is similar to natural ones, so they are also classified as natural.
Figure 4-1-3 Root Amber
Figura 4-1-4 Chihlimbar
Figure 4-1-5 Ivory
Figure 4-1-6 Ivory Artifacts
Figure 4-1-7 mammoth Ivory
Figure 4-1-8 tortoiseshell
Figure 4-1-9 Coral
Figure 4-1-10 Jet
Figure 4-1-11 Antelope Horn
Figure 4-1-12 Ammonite
Figure 4-1-13 Abalone Shell
Figure 4-1-14 Tridacna
Figure 4-1-15 Melo Pearl
Figure 4-1-16 Conch Pearl
Section II Organic gemstones of the Mollusca phylum
1. Pearls
Currently, there are many ways to classify pearls in the market, including freshwater pearls and seawater pearls named after their water sources; cultured pearls and natural pearls named after their origins; black pearls and golden pearls named after their colors; and Japanese pearls and South Sea pearls named after their places of origin, among others. Traditionally, pearls come from the bivalve and lamellibranch classes within the biological category of the Mollusca phylum. This will introduce the commonly seen seawater and freshwater pearls in the market.
1.1 Seawater Pearls
The mainstream seawater pearls on the market currently include South Sea pearls, South Sea gold pearls, Tahitian black pearls, and Akoya pearls.
(1) South Sea pearls
South Sea pearls are a type of seawater pearl produced in the waters of the Philippines, Indonesia, Thailand, Myanmar, Australia, and other regions, with the mother oyster being the pinctada maxima from the class Bivalvia. The Hepu pearls produced by the pinctada martensi in the southern waters of China (Beihai City, Guangxi) also belong to South Sea pearls.
South Sea pearls have a diameter ranging from 8 mm to 20 mm, with an average diameter of about 13 mm. Round to near-round pearls account for approximately 10% ~30% of all South Sea pearls, while oval, flat button-shaped, and drop-shaped symmetrical pearls account for approximately 40%~ 60% of all South Sea pearls, and irregular and semi-irregular pearls account for approximately 20%~40% of all South Sea pearls. The body colors are white, light yellow, and silver, often with yellow, orange, or blue tones, and the overtone are often pink, green, or blue. Among them, white is considered the most precious. The nacre layer is thicker than that of other seawater pearls.
(2) South Sea gold pearls
South Sea gold pearls is an internationally renowned species of South Sea pearls on the northwest coast of Australia, the Philippines, and Indonesia; the mother shell is the gold lip yster shell of mollusk phylum bivalve. Among them, the gold beads produced in Australia are golden, the best color and the gold beads produced in the Philippines are yellow. It is based on people’s love for gold beads. The current pearl optimization treatment technology is also changing with each passing day, and the common optimization treatment methods of gold beads are dyeing treatment, irradiation treatment, and planting colored nuclear treatment.
(3) Tahitian black pearls
Tahitian black pearls, also known as big creek black pearls, are another type of South Sea pearl produced in the coral atolls of French Polynesia in the South Pacific. The mother shell is the black lip shell of mollusk phylum bivalve.
The diameter of Tahitian black pearls ranges from 9 mm to 14 mm, with an average diameter of about 9.5 mm. Round to nearly round pearls account for approximately 40% of all Tahitian black pearls, while oval, flat button-shaped, and drop-shaped symmetrical pearls account for approximately 20% of all Tahitian black pearls. Irregular and semi-irregular pearls account for approximately 40% of all Tahitian black pearls. The body color is black, dark gray, or brown, often with blue to green, purple, or slightly yellow tones, with overtone often pink, green, or blue.
Tahitian black pearls with a black body color and an overtone of peacock green are the most precious.
When selecting in the market, it is important to note that the diameter of Tahitian seawater pearls is generally greater than 8 mm. Pearls smaller than 8 mm can be initially judged as dyed, but final judgment on treatment still requires further verification.
(4) Akoya Pearls
Akoya pearls are a type of nucleated seawater pearl produced in the Seto Inland Sea around Mie, Kumamoto, and Ehime Prefectures in Japan. The mother shell is the pinctada martensi, a bivalve mollusk. The pinctada martensi is also known as the Akoya shell.
On July 11, 1893, in Japan’s Toba, on the island of Sima (now renamed “MIKIMOTO Pearl Island”), Kokichi Mikimoto successfully cultivated the world’s first semi-round pearl and exhibited it at the Columbian Exposition in Chicago, USA. Subsequently, in 1905 (the 38th year of the Meiji era), he successfully achieved the cultivation of round pearls (Akoya pearls). He began researching the cultivation methods for black and white South Sea pearls.
The Akoya pearls cultivated by Kokichi Mikimoto have their own AAA grading standards and pricing system, which are relatively stable overall. This series of measures has created the Akoya pearls, which are internationally renowned internationally. With the company’s promotion of Akoya pearls and the increase in its recognition, the AAA grading standard of Mikimoto Co., Ltd. has been widely recognized internationally. It has become an acknowledged international standard for Akoya pearls.
Agogo pearls have a diameter ranging from 2mm to 11mm, with the most common sizes being 5-9mm on the market. 9-10mm and 10mm+ Agogo pearls are relatively rare. Round to near-round pearls account for about 70%~80% of all Akoya pearls, while irregular and semi-irregular pearls account for about 20%~30% of the total. The body color of Akoya pearls is white or light yellow, often with yellow, pink, or blue hues, and the overtone are often pink or green. Akoya pearls have the strongest luster, earning them the reputation of being like small steel balls.
1.2 Freshwater Pearls
Freshwater pearls refer to pearls produced in rivers and lakes. China is the largest producer of freshwater pearls, with the main farming areas being Zhejiang, Jiangsu, Jiangxi, Hubei, and Anhui. The freshwater pearls are mainly cultivated using the hyriopsis cumingii and the cristaria plicata from the class Bivalvia of the phylum Mollusca.
The diameter of Chinese freshwater pearls ranges from 4 mm to 14 mm. Round to near-round pearls accounts for approximately 2% of all Chinese freshwater pearls, oval and flat button-shaped pearls that are symmetrically shaped account for approximately 2% and irregular-shaped and semi-irregular pearls account for approximately 38%. The body color is white or light yellow, often with yellow, peach, or purple tones, and the overtone are often pink, green, or blue.
In 2013, Zhejiang Jiali Pearl Jewelry Co., Ltd., a core subsidiary of Dongfang Shenzhou Pearl Group, began promoting Edison nucleated freshwater pearls in the market.
Edison pearls possess all the color ranges of conventional freshwater and seawater pearls and feature special metallic iridescence such as deep purple, violet, and bronze. The diameter of the pearls is generally over 11 mm, with a high roundness and fewer surface blemishes.
Whether freshwater or seawater pearls, regardless of the origin of the seawater pearls, all pearls show unique growth lines on their surface when magnified about 70 times (Figure 4-1-17, Figure 4-1-18). These growth lines can help us distinguish between genuine pearls and their imitations. If these lines are filled with pigment, it can be determined as a dyed treatment.
Figure 4-1-17 Growth pattern on pearl surface
Figure 4-1-18 Surface growth patterns and iridescence of pearls
2. Other natural marine pearls
From a biological perspective, the pearls mentioned above belong to the class Bivalvia of the phylum Mollusca. Pearls can also be found in other classes of the phylum Mollusca besides Bivalvia and Scaphopoda. These types of pearls are less commonly circulated in the general market and are more often seen at various auctions, such as melo pearls, conch pearls, and abalone pearls.
2.1 Melo Pearl
Melo Pearl is a type of pearl that does not have a layered structure, produced by the Melo conch (Melo Volutes). This marine gastropod lives along the coasts of South Asian countries such as Myanmar, Indonesia, Thailand, Cambodia, and Vietnam.
The color of Melo pearls ranges from orange-red, orange-yellow, yellow, and yellow-brown to near-white, with the strong orange hue resembling ripe papaya being the most precious. It has a porcellaneous luster, with a surface featuring a special flame structure. There are no visible overtone or iridescence.
The hardness is higher than that of other types of pearls, with a Mohs hardness of about 4.5 to 5. The refractive index ranges from 1.51 to 1.64. The specific gravity is around 2.75. It usually exhibits orange-red or chalky blue fluorescence.
Melo pearls are the largest among all types and are often mistaken for imitations.
2.2 Conch Pearls
Conch pearls, or Kongke pearls, lack a layered structure. They are produced by the Queen/Pink Conch, from the class Gastropoda in the phylum Mollusca, found in Central and South America and the Caribbean. Conch pearls grow inside the sea snail and cannot be farmed artificially. The colors of conch pearls are commonly found between pink and red, with uneven color distribution, some displaying special patterns, porcelain luster, and no visible overtone or iridescence.
2.3 Abalone Pearls
Abalone pearls grow inside the body of abalones from the class Gastropoda in the phylum Mollusca, attached to the single shell of the abalone, flat in shape. They are a type of pearl that is close in color to the inner wall of the abalone shell.
2.4 Keshi Pearls
Keshi pearls, also known as “Kesu pearls,” are a commercial name for a type of pearl that refers to those that are larger in quantity, appear black and white on the surface, and have an irregular and bizarre shape. High-quality keshi pearls are known for their strong pearly luster and rainbow colors, while better-quality keshi pearls are produced in the South Sea.
Section III Light Related to Organic Gemstones Definition of Academic Terms
The optical properties of organic gemstones include color, luster, transparency, luminescence, and special optical phenomenon. Some of these have been explained in the second chapter, so we will not elaborate. This section will briefly discuss the phenomena observed when viewing organic gemstones under lighting conditions and the professional terminology used to describe these phenomena. It is important to note that organic gemstones, like aggregates, do not exhibit color dispersion, pleochroism, or birefringence.
1. The color of organic gemstones
Here, we will describe the color of pearls.
The color of a pearl is a comprehensive characteristic of its body color, overtone and orient, with the body color being the primary description and the overtone and iridescence as supplementary descriptions.
The observation of pearl color is generally done against a gray or white background, avoiding brightly colored objects, using north-facing sunlight or daylight lamps with a color temperature of 5500-7200k at a distance of 15-25 cm from the sample being tested, rolling the pearl to find its body color, and looking for overtones and iridescence in the reflected light from the pearl’s surface.
1.1 Body Color
Body color refers to the color produced by the selective absorption of white light by the pearl and can also be understood as the color of the pearl itself. The uniformity of the pearl’s body color can indicate the thickness of the pearl layers (Figure 4-2-1,Figure 4-2-2).
Figure 4-2-1 Pearl with thin pearl layer (under strong reflected light, the color of the middle and edge of the pearl is greatly contrasted, and the light black gray is different from the color of the pearl body)
Figure 4-2-2 Pearls with thick nacre layer (uniform overall color of pearls under strong reflected light)
The body color of pearls is divided into five series (Figure 4-2-3).
① White series, referring to pearls with body colors such as pure white, creamy white, silver white, porcelain white, etc.
② Red series, referring to pearl body colors such as pink, light rose, light purplish-red, etc.
③ Yellow series, referring to pearl body colors such as light yellow, beige, golden yellow, orange-yellow, etc.
④ Black series, referring to pearl body colors such as black, blue-black, gray-black, brown-black, purple-black, brown-black, iron gray, etc.
⑤ Other series, referring to pearl body colors such as purple, brown, cyan, blue, brown, purplish-red, greenish-yellow, light blue, green, bronze, etc.
1.2 Overtone
Overtones refer to one or several colors that float on the surface of the pearl. The overtones of a pearl may include white, pink, rose, silver-white, or green (Figures 4-2-4 to 4-2-6).
When observing in practice, illuminate the surface of the pearl with reflected light and fix the position of the pearl for multi-angle observation. This phenomenon may sometimes occur near the highlight points of the reflected light.
Figure 4-2-4 Black pearls, with overtone from left to right being light pink, powder blue, light green, and light purple.
Figure 4-2-5 On the left is a white pearl with a light pink overtone, and on the right is a red pearl with a white overtone that shows a visible halo effect at the top.
Figure 4-2-6 Yellow pearls: the two on the left are natural golden pearls with an inconspicuous light green overtone; the two on the right are dyed golden pearls with an almost invisible overtone.
1.3 Iridescence
Iridescence refers to the iridescent rainbow colors that can drift on or just below the surface of pearls (Figure 4-2-7). The description of orient does not require detailing its colors, only its intensity, which is usually indicated by four levels: strong (Figure 4-2-8), obvious (Figure 4-2-9), general (Figure 4-2-10), and not obvious (Figure 4-2-11, Figure 4-2-12)).
Figure 4-2-7 The iridescence of pearls.
Figure 4-2-8 Pearls with strong iridescence (the largest irregular pearl).
Figure 4-2-9 Pearls with obvious iridescence
Figure 4-2-10 Pearls with general iridescence.
Figure 4-2-11 The upper part of this irregular pearl has an inconspicuous iridescence, while the lower part shows a distinct iridescence.
Figure 4-2-12 Pearls with subtle Iridescence
2. The Luster of Organic Gemstones
There are eight types of gemstone luster discussed. In previous articles, we have already covered the four types commonly seen in crystals: metallic luster, adamantine luster, glassy luster, and oily luster, and have discusses oily luster, silky luster, and waxy luster. The clusters found in organic gemstones include pearly luster and resinous luster.
2.1 Pearly Luster
Light-colored transparent minerals exhibit a soft and colorful luster on their perfectly developed cleavage surfaces, similar to the surface of pearls or the inner walls of shells, such as muscovite and selenite (Figures 4-2-13, 4-2-14).
Figure 4-2-13 Pearly Luster (Muscovite Cleavage Surface, Reflected Light)
Figure 4-2-14 Pearly Luster (Pearl, Reflected Light)
When observing pearls, there are specific classifications for evaluating their luster. (Table 1, Table 2). Generally speaking, seawater pearls’ luster is stronger than freshwater pearls (Figure 4-2-15, Figure 4-2-16).
Table 1: Luster levels of Seawater pearl
| Gloss level | Quality Requirements | |
|---|---|---|
| Chinese Description | English Code | Quality Requirements |
| Extremely strong | A | he reflected light is particularly bright, sharp and even, the surface is like a mirror and the reflection is very clear. |
| strong | B | The reflected light is bright, sharp, and even, with a clear image. |
| Medium | C | The reflected light is bright, and the surface can show the object's image. |
| Slab | D | The reflected light is weaker, the surface can reflect the object, but the image is somewhat blurry. |
| Note: The luster level of gem-quality seawater pearls is at least medium (C). | ||
Table 2: Luster levels of freshwater pearl
| Gloss level | Quality Requirements | |
|---|---|---|
| Chinese Description | English Code | Quality Requirements |
| Extremely strong | A | The reflected light is bright, sharp, and uniform, and the image is very clear. |
| strong | B | The reflected light is bright, and the surface can see the object's image. |
| Medium | C | The reflected light is not bright, the surface can reflect the object, but the image is relatively blurry. |
| Slab | D | The reflected light is diffuse; the surface has a dull sheen and almost no reflection. |
| Note: The luster level of gem-grade freshwater pearls is at least medium (C) | ||
Figure 4-2-15 Strong luster seawater pearls (highlights are clear and sharp at the edges)
Figure 4-2-16 Pearl luster comparison (from left to right, the first column is Japanese pearls, the second, third, and fourth columns are seawater pearls, and the far right column is freshwater pearls)
2.2 Resinous Luster
In mineralogy, resinous luster is defined as follows: on the uneven fracture surfaces of certain yellow, brown, or transparent minerals with a adamantine luster, a resin-like sheen can be observed, such as in light-colored sphalerite and realgar.
In organic gems, the gemstone often exhibiting a resinous luster is tortoiseshell, while fossilized resins such as amber, beeswax, and copal resin are also included (Figure 4-2-17, Figure 4-2-18). From practical identification, the strength of the resin luster at the fracture can effectively distinguish between amber and copal resin. (Figure 4-2-19).
Figure 4-2-17 Resinous luster tortoiseshell
Figure 4-2-18 Resinous luster amber
Figure 4-2-19 Comparison of resinous luster at the fracture of amber (left) and copal resin (right), the resinous luster of amber is stronger than that of copal resin.
3. Transparency of Organic Gemstones
The terminology used to describe organic gemstones’ transparency is consistent with crystal transparency. Still, it is necessary to point out separately when the transparency of organic gemstones is uneven (Figures 4-2-20 to 4-2-23).
Figure 4-2-20 Transparent amber
Figure 4-2-21 Translucent antelope's horn
Figure 4-2-22 Micro-translucent pearl
Figure 4-2-23 Opaque jet
The texture of transparent to semi-transparent organic gemstones (the phenomenon of overlapping transparency and structure) can sometimes serve as important evidence for distinguishing their types, such as amber and copal resin (Figure 4-2-24).
4. Luminescence of Organic Gemstones
Except for blue amber (Figure 4-2-25), the luminescence of organic gemstones is generally not observable to the naked eye.
However, fluorescence phenomena can be easily seen under ultraviolet fluorescent light. A special note for organic gemstones is the uniformity of the fluorescence description, as the fluorescence of organic gemstones like Amber is generally uneven under ultraviolet light (Figure 4-2-26).
5. Special Optical Phenomenon of Organic Gemstones
The halo effect of pearls is common in organic gemstones, and other special optical phenomenon are rare.
The halo effect of a pearl is the iridescent color that can drift on or under the surface of the pearl.
The organic gemstones where the halo effect can be observed include pearls (Figure 4-2-27), abalone shells, and ammonite (Figure 4-2-28)
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Section IV Fossil organic gemstones
1. Amber
1.1 Formation of Amber
Amber is an organic mixture formed from the resin of coniferous plants from the Mesozoic Cretaceous to the Cenozoic Tertiary period through geological processes. The formation of Amber generally has three stages: the first stage is the secretion of resin from the pine tree; the second stage is when the resin is buried deep and undergoes fossilization, resulting in significant changes in its composition, structure, and characteristics; the third stage is when the fossilized resin is washed, transported, deposited, and undergoes lithification to form Amber.
1.2 Classification of Amber
According to the national standard “GB/T 16553-2010 Gemstone Identification,” Amber is divided into beeswax, succinum, golden amber, green amber, blue amber, worm amber and plants amber .
Beeswax refers to translucent to opaque amber (Figure 4-2-29, Figure 4-2-30). Blood red amber refers to red to red transparent amber (Figure 4-2-31). Golden amber refers to yellow as golden transparent amber (Figure 4-2-32). Green amber refers to light green to green transparent amber, which is relatively rare. Blue amber refers to the perspective observation of amber body color for yellow, brown, yellow, green, brown, red, and other colors, showing unique shades of blue under natural light, more obvious under ultraviolet light. Mainly produced in the Dominican Republic (Figure 4-2-25), Mexico (Figure 4-2-33), etc. A worm amber is an amber-containing insect or other living thing. Plants amber are amber-containing plants (flowers, leaves, roots, stems, seeds, etc.).
Figure 4-2-29 Beeswax
Figure 4-2-30 Beeswax
Figure 4-2-31 Blood red amber
Figure 4-2-32 Golden amber (from left to right, the third is worm amber )
1.3 Common Internal Features of Amber
Common internal features of Amber include bubbles, flat fissures (Figure 4-2-34), flowing fissure patterns (Figure 4-2-35), flow patterns (Figures 4-2-36, 4-2-37), mineral inclusions, and inclusions of animals and plants (Figure 4-2-38), gas-liquid two-phase inclusions (Figure 4-2-39), etc.
Figure 4-2-34 Flat fissures in Amber (Dark Field Illumination, 40X)
Figure 4-2-35 Flowing texture of fissures in amber (dark field illumination method, 40X)
Figure 4-2-36 Flow Patterns in Amber (Dark Field Illumination, 20X)
Figure 4-2-37 Flow lines of beeswax
Figure 4-2-38 Insects in Amber (Dark Field Illumination, 40X)
Figure 4-2-39 Gas-liquid two-phase inclusions in amber (dark field illumination method, 20X)
1.4 Main Imitations of Amber
Common imitations of Amber include two main categories: natural resin types and plastics.
Natural resins are classified based on their curing time and whether they have undergone geological processes, with the duration of geological processes ranging from short to long, divided into hard resin, colophony, copal resin, and amber. Among them, hard resin, colophony, and copal resin (Figure 4-2-40) are common natural imitations of amber.
Plastic is a common artificial gemstone imitation of Amber (Figure 4-2-41). Plastic imitations of Amber can be distinguished from several aspects, such as the pattern of flow lines (Figures 4-2-42 and 4-2-44) and fissure. (Figure 4-2-45).
Figure 4-2-40 Copal resin
Figure 4-2-41 Plastic
Figure 4-2-42 Flow Patterns of Plastic (Vertical Illumination Method, 30X)
Figure 4-2-43 Flow Patterns of Plastic (Dark Field Illumination Method, 10X)
Figure 4-2-44 Flow Patterns of Plastic (Dark Field Illumination Method, 10X)
Figure 4-2-45 Fissures in Plastic (Dark Field Illumination, 40X) The most famous variety is blue Amber with a greenish tint.
1.5 Main Production Areas of Amber
In the market, Amber is commonly divided into sea amber, rock amber, and coal amber based on the geological environment of its origin. According to the place of origin, Amber is divided into Russian Amber, Ukrainian Amber, Burmese amber, and Mexican Amber.
① Baltic Sea Coast
Among the many countries along the Baltic Sea, the famous amber-producing countries are Ukraine, Poland, Lithuania, and Russia.
The amber deposits along the Baltic Sea coast are distributed in coastal areas, some extending underwater. As the waves erode the deposits, Amber is stripped away. Due to its relatively lower density than seawater and high transparency, it can float on the water’s surface. It is known as sea amber, also called sea stone.
② Myanmar.
Myanmar amber is produced in northern Myanmar’s Hukang Valley of Kachin State. It belongs to deep-seated deposits that are difficult to mine, and common varieties from this source include golden Amber, root amber (Figure 4-2-46), and brown Amber.
③ Fushun amber producing area.
Fushun amber is produced in the open-pit coal mine in Fushun, Liaoning, China. The Amber with impurities belongs to coal amber. This is the only source of gem-quality amber and insect amber in China.
④ Dominican Republic
Dominican blue Amber is produced in the Greater Antilles of the Caribbean. The most famous variety from this source is blue Amber.
⑤ Mexico amber production area.
Mexican Amber is produced in the southeastern state of Chiapas, Mexico. This source
2. Mammoth Ivory
The mammoths was a large mammal that became extinct about 12,000 years ago. Mammoths lived in herds on the Siberian plateau, and a portion of them that were buried alive due to geological changes at least 10,000 years ago still have their tusks that have not turned into fossils or semi-fossils.
Mammoth ivory, also known as ancient ivory, refers to the upper incisors and molars of the ancient mammal mammoths that have not completely fossilized, belonging to the remains of prehistoric life. They are mostly preserved in the permafrost layers of places like Siberia and Alaska. The former is mainly found in the Lena River and other rivers flowing into the Arctic Ocean, while the latter has been found in the Yukon River basin in Alaska.
Mammoth ivories are long and curved upwards, and most mammoth ivory can no longer be used for carving, with a yield rate of only about 20%. High-quality fossil ivory can be as beautiful as regular ivory. Some fossil ivory that has been stained blue or green by iron copper phosphate is referred to as “odontolite” and can be used as a substitute for ivory, with most materials imported from Siberia.
Currently, the processing of mammoth ivory has formed its unique style, with some mammoth ivory crafts retaining the ‘ivory skin,’ highlighting an ancient and solemn style (Figure 4-2-47).
3. Jet
Jet, also known as gageite, is a special type of coal formed from the hard trees rich in oil from ancient forests that were washed to low-lying areas by floods, undergoing geological changes, high temperatures, and the modification process under underground pressure, resulting in a black crystalline substance. The formation of jets must come from lush plants that grew during a certain geological era, gradually accumulating into thick layers in a suitable natural environment and being buried underwater or in sediment, followed by a long geological period of natural coalification.
Coal essence, with a bright pitch and metallic luster, black, dense, highly tough, and a streak color of chocolate. It is lighter than ordinary coal. Coal essence can be used to make arts and crafts, carved artworks, and decorations. Hence, some people refer to it as carved lacquer coal. The production areas of coal essence include China, Chile, and Germany. The best coal essence in China comes from Fushun City, Liaoning Province, and is one of Liaoning’s unique craft gemstones.
In addition, there is a coal root stone, a petrochemical phenomenon of coal. It is grayish-black with bluish-black, one of the traditional seal stones. The color and brightness of coal root stones are not as good as coal essence but are slightly stronger than coal essence in seal cutting, one of the rare and rare seal stones treasured by collectors.
4. Ammonite
Ammonite is a species of fossil in the Daisy family, which can reach gemstone grade and are mainly characterized by a gorgeous color-changing effect (Figure 4-2-48). Ammonite is produced in Canada, Madagascar, the United States, and the United Kingdom, etc., among which Canada’s multi-color cameras are suitable for processing, and can be cut down to be made into finished gems, the multi-color cameras seen in China’s mainland are mainly from Madagascar, suitable for viewing and playing.
The reason for ammonite’s color change effect is not the ammonit’s opal transformation. The surface layer of ammonite is the calcite phase, and the surface layer is the aragonite phase. The color change effect is limited to the surface layer, which disappears after the surface layer is destroyed. The color change effect of ammonite is the interference effect on visible light caused by the thickness change of the surface patch of calcite, which is a component of the stylolite as the boundary. With the movement of gemstones, the incidence Angle of light changes, and the optical path difference of the interfered light changes accordingly, so the color generated by the interference changes.
Section V Interpretation of Mechanical Properties Related to Organic Gemstones
The mechanical properties of gemstones are divided into 7 phenomena categorized into 4 types: cleavage, parting, and fracture belong to one category, while the other three categories are hardness, density, and toughness. Here, we will discuss breakage, hardness, and relative density related to organic gemstones.
1. Fracture of Organic Gemstones
Common shell-like fracture in organic gemstones (Figures 4-3-1 to 4-3-3)
Figure 4-3-1 Different Patterns of Amber Shell-like Fractures
Figure 4-3-2 Different patterns of amber shell-like fractures
Figure 4-3-3 Different Patterns of Amber Shell-like Fractures
2. Hardness of Organic Gemstones
The hardness of organic gemstones is between 2 and 7, which makes organic gemstones easy to process, but also makes organic gemstones need to pay attention to avoid contact with other harder substances in the process of wearing and maintenance in the later period to avoid surface damage of organic gemstones.
3. Relative Density of Organic Gemstones
The relative density of organic gemstones varies greatly due to differences in composition; for example, the density of pearls ranges from 2.60 to 2.85, while that of tortoiseshells is only 1.29.
It should be noted that Amber, with a density of 1.32 and no visible inclusions to the naked eye, usually floats on saturated saltwater. This is the simplest way to distinguish Amber from most plastic imitations. This method does not apply to Amber with inclusions (Figure 4-3-4).
Section VI Coral
Coral polyps are marine tubular coelenterates that automatically attach to the calcareous remains of ancestral corals in their white larval stage.
Coral is the shell secreted by coral polyps, and its chemical composition is mainly calcium carbonate, existing in the form of microcrystalline calcite aggregates. It also contains some organic matter, typically appearing in a branching form with longitudinal stripes. Each coral cross-section has concentric and radial stripes. Coral and coral reefs are two different varieties.
Gem-quality coral is also known as red coral, and it is divided into two types based on composition: horny coral and calcareous coral.
1. Horny Coral
Horny coral is mainly composed of organic matter. Common colors include black, gold, and blue, with a density of around 1.34 g/cm³, which is rare in the market (Figure 4-3-5).
2. Calcareous Coral
The composition of calcareous coral consists of calcium carbonate and organic matter content not exceeding 7%. Common colors include red, pink, orange-red, white, blue, and gold, with a density between 2.6~2.7 g/cm³ (Figure 4-3-6).
(1) Akka Red Coral
Its full name is ” Chiaka Coral” (Figure 4-3-7). “Aka” is the Japanese pronunciation for “red,” and “Chiaka” is the pronunciation for blood red, which is transliterated into Chinese as Akka. Akka red coral grows in Japan and a small part of China Taiwan.
After the Black Ship Incident in 1853, Japan’s Akka red coral was forced to open its doors and was sold to Europe by Westerners. This high-quality coral is known as Akka red coral. Akka red coral refers to high-quality coral produced in Japan. Thus, there is a price difference between Japanese Akka and China Taiwanese Akka.
The best color of Akka red coral is oxblood red, but the vast majority of raw Akka red coral has uneven color distribution and contains white cores. The white core is a white part at the center of the coral branch, resembling ivory. This is one of the most important characteristics distinguishing Akka red coral from other red corals.
Because Akka red coral grows in deep water under the sea surface, the shape of the coral branches is not a circular cylindrical cross-section, but the front is slightly flat, the back is curved, and the plants are small. It is precisely because they live in deeper waters that Akka red coral has been under a lot of pressure in the deep sea, and internal stresses in the coral are resistant to external pressures. When Akka red coral is fished out of the sea, the external pressure is reduced, the internal stress is released, and deep or shallow cracks are formed. Red corals like Sardines and Momo corals have rare stress patterns.
Akka red coral has a front and back; generally, the front is red, with a smooth texture (good translucency) and good luster, while the back has more imperfections and wormholes.
Polished Akka red coral has a glass-like translucent quality and texture, giving it a sparkling and slightly transparent appearance, with the characteristic concentric and radial stripes of coral being less pronounced (Figure 4-3-8).
Figure 4-3-7 Akka coral
Figure 4-3-8 Comparison of cross-sections of Akka coral.
(2) Momo red coral
The Japanese name for peach coral is pronounced as “Momoirosango,” abbreviated as MOMO, which transliterates to Chinese as Momo (Figure 4-3-9).
The Momo family is a large and complex classification within corals; aside from Akka and Sardines, the rest can be classified into the Momo family.
Momo coral is mainly produced in the waters off China Taiwan, and its colors are also very rich, including shades of pink, peach, light pink, and orange, as well as white cores. Overall, the colors of momo coral are mostly light shades of red, with deep red and bright red being less common. If colors are close to those of the Akka coral, they can only be called Akka-grade coral rather than Akka coral.
Notable varieties of Momo Coral include Blood Peach Coral, “Child’s Face,” “Phoenix,” and SUKACHI.
Blood Peach Coral: A type of Momo Coral with colors and quality similar to Akka, generally a deeper red with orange or yellow tones.
“Child’s Face,” also known as “Angel Skin,” in Japanese is Hon Boke; in English, Angel Skin. It refers to deep-sea coral that is pink and evenly colored.
“Phoenix,” in Japanese is MAGAIBOKE, in English Phenix, compared to ” Child’s Face” the color is slightly darker, with a gradient that becomes deeper and more uneven.
Coral with many white spots is called SUKACHI.
Momo coral has a texture between Akka and Sardin, but the texture of Momo coral is closer to Akka. Unlike Akka coral, Momo coral has a porcelain-like solid texture, and the unique concentric and radial stripes of the coral are clearer (Figure 4-3-10).
Figure 4-3-9 Momo Coral
Figure 4-3-10 Comparison of cross-sections of momo coral.
(3) Sardin Red Coral
Sardin coral is called deep-water coral and grows near Sardinia, Italy, because most of the operators are Italian; it is also called “Italian coral” (Figure 4-3-11). With the change of the coral era, Sardin coral is generally called deep-water coral in the Mediterranean Sea. It is mainly produced in the waters near Sardinia in the Mediterranean Sea. In the past, people called the coral produced in Sardinia, Italy, “Sardin coral.” However, now Sardin coral is regarded as a species, which means that as long as the color, hardness, etc., are within a certain range in a certain area, it can be called Sardin coral. Sardin coral generally grows 50m to 120m below sea level, one of the shallower growth regions of all red corals, so stress lines are rarely seen.
The color of Sardin coral is similar to that of Akka, commonly seen in orange, pink-red, vermilion, bright red, and deep red. However, it can achieve the deepest colors of Akka coral. The overall characteristic of Sardin coral is its uniform, rich red color without a white core. The coral jewelry products commonly seen in the market, such as bracelets and necklaces, are mostly made from this material (Figure 4-3-12).
Sardin coral has the smallest density among several precious corals, making it relatively loose. Therefore, it does not have the fine texture and good clarity of cut and polished Akka coral and Momo coral, and it is prone to turning white, darkening, and fading (Figures 4-3-13, 4-3-14).
Figure 4-3-11 Sardin Coral
Figure 4-3-12 Comparison of horizontal and vertical cross-sections of Sardin Coral
Figure 4-3-13 Akka Coral (Left one and Left 2), Sardin Coral (Left 3), Momo Coral (Right one and Right 2 ) ) texture comparison
Figure 4-3-14 Comparison of the growth ring visibility of Sardin Coral, Momo Coral, and Akka Coral
Section VII Rare Organic Gemstones
1. Tridacna
Tridacna (Figure 4-3-15) is the collective name of the organisms belonging to the order bivalvia, family tridacna, which has two genera and ten species. They are widely distributed in tropical coral reef waters. According to the “China Marine Mollusk Atlas” published in 2003, there are six species distributed in China, including the tridacna gigas, the tridacna derasa, the tridacna squamosa, the tridacna maxima, the tridacna crocea, and the hippopus hippopus. Of these, five have shell lengths of up to 50cm. The tridacna gigas is the largest bivalve species, with the largest recorded individual having a shell length of 1.3m, weighing 500kg, and an age of over 60 years. It also exhibits great superiority in growth, with one specimen reaching a shell length of 40cm and a weight of 15kg. Tridacna are mainly distributed in the Indian Ocean and Pacific Ocean. They are found in low-tide zones near coral reefs or shallow reefs in Indonesia, Myanmar, Malaysia, the Philippines, Australia, and other countries. They are also widely distributed in Hainan Province, China Taiwan Province, and the South China Sea Islands.
Among the ten types of Tridacna, the Tridacna gigas, also known as Koo’s clam, is a nationally protected species at the first level and is listed in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) as a Class II species. The scaly clam is a nationally protected wild animal at the second level. Other species are not mentioned.
Tridacna is often cut and polished into spherical shapes in the market, with holes drilled along a certain diameter (Figure 4-3-16), and is used to make bracelets or necklaces. The uneven transparency of Tridacna (Figure 4-3-17) and the special texture on the surface when polished are important features that distinguish it from imitations made of Glass, plastic, etc. (Figure 4-3-18).
Figure 4-3-15 Tridacna
Figure 4-3-16 Golden Tridacna
Figure 4-3-17 The uneven transparency of the Golden Tridacna (10X, dark field illumination method
Figure 4-3-18 Surface pattern of Golden Tridacna ( 40X , vertical illumination method)
2. Ivory
In a narrow sense, Ivory refers to the tusks of male elephants, which are often processed into artworks, jewelry, or gems (Figure 4-3-19). It is also processed into billiard balls and piano keys, making it a very expensive raw material. Teeth and tusks are the same material. Teeth are specialized structures used for chewing. Tusks are elongated teeth that extend beyond the lips; they evolved from teeth and are generally used as defensive weapons. In 1973, representatives from 21 countries were mandated to sign the Convention on International Trade in Endangered Species of Wild Fauna and Flora in Washington to protect elephants from being killed, which strictly limits the ivory trade. China joined this Convention in 1981. Although elephants replace their teeth six times in their lifetime, due to the high demand for Ivory in Asian culture, there are numerous illegal poaching incidents in elephant habitats to obtain Ivory. Furthermore, since the ivory trade is a significant economic source for some African countries, and to preserve the traditional culture of ivory carving in Asia, the Convention approved China and Japan as legal ivory-importing countries in 2008.
Figure 4-3-19 Ivory bracelet
Figure 4-3-20 Ivory Structure
The cross-section of Ivory is typically divided into four layers from the outside to the inside (Figure 4-3-20):
- Coarse concentric layer, relatively thin, only 0.5-3 mm.
- Coarse reticulate layer, dentin, which has significant identification meaning, with the maximum angle between the two groups of textures pointing to the tooth center greater than 120° (Figure 4-3-21) and the average angle greater than 110°. From the root of the Ivory to the tip, the angle of the coarse reticulate texture gradually decreases, with a wider spacing between texture lines, measuring 1-2.5 mm.
- Fine reticulate layer, where the angle between the two groups of textures pointing to the tooth center gradually becomes smaller, generally less than 90°, with a very narrow spacing between texture lines, measuring 0.1-0.5 mm.
- Fine concentric layers with cavities.
On the longitudinal section of Ivory, a group of subtly visible wave-like textures distributed nearly parallel and discontinuously (Figure 4-3-22).
The characteristic Lutz texture of Ivory is an important feature for distinguishing between Ivory, mammoth ivory, ivory nuts, plastics, and other imitations.
Mammoth ivory (Figure 4-3-23) has a cross-section that shows a concentric layered growth structure similar to that of Ivory (Figure 4-3-24), but the differences are: the thickness of the coarse concentric layer (Layer A) is relatively large, with locally developed “V”-shaped fissure; in the coarse Lutz layer (Layer B), the angle between the two groups of textures pointing towards the dental core is relatively small, with the maximum angle being less than 95°. In the longitudinal section, the wave-like texture of mammoth tusks could be clearer, with visible linear textures (Figures 4-3-25, 4-3-26).
Figure 4-3-21 The coarse and sparse ivory layer angle is greater than 120°.
Figure 4-3-22 The microwave-like texture of the Ivory longitudinal section is intermittently distributed nearly parallelly.
Figure 4-3-23 Mammoth Ivory
Figure 4-3-24 Mammoth Ivory Structure
Figure 4-3-25 Coarse concentric layer patterns near the "dentin" area on the side of mammoth ivory
Figure 4-3-26 The maximum angle of mammoth ivory is less than 95° The intersection angle's Lutz texture (above) and the linear texture on the longitudinal section (below)
3. Skull of the helmeted bird
The helmeted hornbill is a tropical bird from the Old World, belonging to the order Bucerotiformes, family Bucerotidae, and genus Rhinoplax. Its skull resembles a helmet, sitting atop a prominent casque. It is found in lowland forests below 500m in southern Myanmar, Thailand, the Malay Peninsula, Borneo, and Sumatra. Its solid casque, red on the outside and yellow on the inside, is fine-textured and easy to carve, comparable to Ivory. It is often made into various crafts, widely collected, and known as “crane top red.”
4. Tiger teeth and tiger bones, antelope horns, and rhino horns
In the face of the disappearance of habitats for Asian elephants and the reduction of imported Ivory, tiger teeth, tiger bones, antelope horns, and rhino horns have become one of the substitutes for Ivory in the Ivory carving industry.
Tiger teeth are the upper canines of the tiger, a felid, which are dark white and elongated, with thick roots. An adult tiger has only four canines, two in the upper jaw and two in the lower jaw. Due to the excessive hunting of tigers by humans and the unreasonable development of their natural habitats, the number of tigers has decreased, and their wild habitats have shrunk, making tigers a rare and endangered species, classified as a national first-class protected animal.
Antelope horn, the horn of the male bovine species Saiga Tatarica. Distributed in the border areas of northwestern Xinjiang. The Saiga antelope has been listed in the IUCN Red List of Threatened Species 2012 ver3.1 as Critically Endangered (C.R.), and hunting is strictly prohibited.
Rhino horn, also known as rhino horn, is the horn of rhinoceros species such as the Indian, Javan, and Sumatran rhino.
