If beryl contains chromogenic ion Cr, its color is emerald.
The beryl containing chromogenic ion Fe is sky blue or sea blue, which is called aquamarine.
The beryl containing chromogenic ions Cs, Li and Mn is rose red, which is called cesium beryl, and its English name comes from the name of American gem lover (JPMorgan Chase).
Fe and beryl with golden yellow and light lemon yellow are called heliodor, and the English name comes from the Greek word "Sun".
The beryl containing titanium and iron is dark brown, which is called dark brown beryl.
Emerald is the most important and precious variety of beryl, and is called "the king of green gems" by the world. Together with diamonds, rubies, sapphires and cat's eyes, it is known as the "five treasures" endowed by nature. Emerald is green and pleasing to the eye. As the birthstone of May, its color represents the arrival of spring.
I. Basic nature
Crystallographic characteristics
Crystal system hexagonal system.
Crystallization habit crystals are hexagonal columns, mainly composed of hexagonal columns, hexagonal biconical cones and parallel double faces (Figure 6-8).
Figure 6-8 beryl crystal
Surface features There are common horizontal lines on the surface, hexagonal corrosion marks on the axial surface and rectangular corrosion marks on the cylindrical surface.
(2) Chemical composition
Beryllite is beryllium aluminum silicate, and its chemical molecular formula is be3al2 [si3o 18], which can contain trace elements such as chromium, iron, lithium, manganese, vanadium and iron, and can be divided into different gem types.
(3) Physical characteristics
1. Optical characteristics
Colors vary widely, such as green, blue, yellow, black, red and colorless, which determine the variety.
Gloss is generally glass luster.
Transparent, transparent and opaque.
Refractive index 1.56 ~ 1.59, depending on the variety.
The birefringence is 0.004 ~ 0.009, depending on the kind of gem.
Optical uniaxial crystal, negative optics.
Low dispersion (0.0 14).
Polychromaticity is obvious, depending on the variety.
Absorption spectrum Emerald has a typical absorption spectrum, with clear spectral lines at 683nm, 680nm and 637nm in the red region, weak universal absorption at 630 ~ 680nm, and weak spectral lines at 477nm in the blue region. Aquamarine is colored by iron, and its absorption spectrum is not obvious. There is a slightly wider band at 427nm in the purple region and a weak line at 456nm in the blue-purple region. Sometimes light green aquamarine shows a spectral line at 537 nm.
Optical effect beryl gemstones can present starlight and cat's eye effects.
2. Mechanical characteristics
Cleavage is incomplete and parallel to the bottom of the crystal.
The fracture is shell-shaped.
The hardness is 7.25 ~ 7.75, and the emerald is fragile.
The density is 2.7 ~ 2.9g/cm3, depending on the kind of gemstone.
(4) Inclusion
There are rich inclusions in beryl gems, and there are obvious differences among different gem varieties and among the same gem varieties from different places. For example, Colombian emeralds have typical three-phase inclusions, Russian emeralds have bamboo-like actinolite inclusions, and Indian emeralds have comma-like inclusions. However, aquamarine and other beryl varieties have typical rain-like inclusions (Figure 6-9). Inclusion is the most important basis for identifying the variety and origin of beryl gems.
Figure 6-9 Rainy Filamentous Inclusions in Aquamarine
Two, the main gem varieties and their identification and evaluation
1. Emerald
Emerald, like many other gems, has a long and legendary history. Legend has it that the Holy Grail used by Jesus at the Last Supper was carved from jade. Emerald is also mentioned in the Bible, and its Song of Solomon says, "Women of Jerusalem, this is my love and this is my friend! His hand is like a gold ring set with emeralds. " According to historical records, as early as 6000 years ago, emeralds were sold in the market in Babylon, Cuba. In the kingdom of Chaldea (New Babylon), women especially like to wear emerald ornaments. In ancient Greece, emeralds were called "emeralds" and "glowing gems", which were noble treasures dedicated to Venus, the goddess of love and beauty in Greek mythology. In 2000 BC, the jadeite mine in Wadi Sisat, a mountainous desert 440 miles southeast of Cairo and 25 miles west of the Red Sea, was mined. According to legend, Cleopatra not only often wore emerald jewelry more than 2,000 years ago, but also named the emerald mine after her. Some amulets and beads made of emeralds in this mine have been passed down to this day and become priceless. The mine was owned by Egyptians, Greeks, Romans and Turks from ancient times to18th century, and abandoned in 1740. Later, the exact location was unknown. But in 1770, Scottish explorers discovered it while crossing the desert, tried to exploit it again, and finally gave up because of the exhaustion of resources. Ancient crude oil exploitation left caves, tunnels and tunnels that crossed mountains for miles. The ancient Romans also liked emeralds. Nero, the Roman tyrant, wore glasses made of emeralds. At the same time, Nero also relied on his power to look for jade everywhere. Ancient Persians also liked jade. According to incomplete statistics, there are thousands of emeralds in the Iranian royal family alone. In ancient India, most legends often associate emeralds with Africa, and the gold or silver rings made by ancient Indians according to Navaratne style are also inlaid with emeralds. In the Inca Empire, the king's pure gold crown was inlaid with 453 emeralds, weighing * * * 152 1ct, the largest of which was 45ct, 1593. The crown was placed on the statue of the Virgin in Columbia Cathedral. Others, such as the king of Antioch and the king of the British Empire, have a lot of emeralds in their crowns.
Emerald is the most precious member of beryl family and one of the most precious gems in the world. It is known as the king of green and is deeply loved by people. Since ancient times, emeralds and diamonds, rubies, sapphires and opals have been regarded as the five precious stones in the world. Emerald is green and pleasing to the eye, and it is fascinating to all ages. It is appropriate to choose this lovely beryl gem as the May Birthday Stone. Its color represents the beauty of nature and a symbol of commitment when spring comes. At the same time, it is also a symbol of loyalty, kindness and kindness. A long time ago, emerald was used as a gem for Venus, the goddess. Lovers thought it had the magic power to reveal the loyalty of their loved ones.
In addition to the common emeralds, emeralds can be divided into the following three varieties according to the varieties with special optical effects and special phenomena.
(1) Emerald Cat's Eye
Emerald can produce cat's eye effect (Figure 6- 10) because it contains a group of tubular inclusions arranged in parallel and densely distributed, but it is not common.
Figure 6- 10 Jade Cat's Eye
(2) Starlight Emerald
Starlight emeralds are extremely rare, only occasionally found. In addition to the tubular inclusions parallel to the Z axis, there are unknown particles in two directions, one of which is perpendicular to the Z axis.
(3) Trapiche
This is a special type of emerald, which is produced in Mzo region and Chival region of Colombia. It has special growth characteristics. Darbiz produced by Mzo has a dark core and radial arms in the middle of green emeralds, and is composed of carbonaceous inclusions and albite, sometimes calcite and pyrite are rare. X-ray diffraction proves that this Darbitz is a complete single crystal. The Darbiz Emerald produced by Chival has a green hexagonal core with six green arms protruding from the hexagonal prism of the core. In the V-shaped area between the two arms, there is a mixture of albite and emerald. X-ray diffraction proves that this piece of Dahlby emerald is also a single crystal, and albite is wrapped in the emerald crystal.
2. Economic evaluation
The evaluation of jadeite is mainly based on the origin, color, transparency, purity and weight.
Colombian emeralds have the best producing area, and the high-quality emeralds with 0.2 ~ 0.3 CT can be used as high-grade jewelry rings. 0.5ct quality is more expensive than diamond, followed by Tanzanian emerald, which can be compared with Colombian emerald. The price of emeralds produced in other areas depends on the specific quality.
The greater the weight, the more precious it is.
Figure 6- 1 1 Darbitz
Color, transparency and clarity can be used as comprehensive indicators. According to this comprehensive index, emeralds can be divided into three grades.
Grade I: dark emerald or blue-green, transparent, with few inclusions, and the crack does not exceed 5% of the total volume;
Grade II: the color is turquoise or blue-green, transparent, with few inclusions, and the crack does not exceed10% of the total volume;
Grade III: the color is green with blue, translucent-transparent, less wrapped, and the crack does not exceed 20% of the total volume.
identify
Emerald includes imitation, synthetic emerald, optimized product and identification of origin.
Identification of (1) imitation
Among all kinds of gems, there are many gems that can imitate emeralds, but their physical characteristics are quite different. By testing various physical characteristic parameters, it is not difficult to distinguish them.
Chrome diopside is usually dark green to yellow green, and reacts to red under the color filter, especially chrome diopside produced in Urals Mountains, which is easily sold as emerald in the market. The difference between emeralds and chrome diopside mainly lies in color and refractive index. Both colors can be dark green to yellow green, but the color of chrome diopside is not as soft as emerald and a little dark. Its refractive index is1.675 ~1.701,its birefringence is 0.024 ~ 0.030, and its density is 3.29g/cm3. Its visible light absorption spectrum also has chromium absorption in red light region, but there is an absorption line at 505nm. There may also be gas-liquid inclusions and tubular inclusions in chrome diopside, but there are few three-phase or two-phase inclusions common in emeralds.
The color of Cr-V-Ca-Al garnet is yellow-green to bright green, and its surface luster is stronger than emerald. Like emeralds, Cr-V-Ca-Al garnet is red or pink under the color filter, and has chromium absorption spectrum in the red region. The difference between emerald and Cr-V-Ca-Al garnet is mainly in optical characteristics, polychromatic, refractive index, density and so on. Cr-V-Ca-Al garnet is isotropic, polychromatic and has a refractive index as high as 1.740. The density is higher than that of emerald, about 3.6 1g/cm3, and there are no three-phase or two-phase inclusions in emerald. The common inclusions in Cr-V-Ca-Al garnet are solid inclusions or negative crystals.
Garnet is green to dark green, and its characteristics are basically similar to Cr-V-Ca-Al garnet. It is isotropic, polychromatic, with a refractive index as high as 1.888 and a density of 3.84g/cm3. There are common horsetail asbestos mineral inclusions.
Green tourmaline is blue-green to dark green, with a dark tone. Its refractive index, birefringence and density are higher than that of emerald, which are 1.62 ~ 1.64, respectively. Its density is 3.06 (+0.02, -0.06)g/cm3, and its birefringence is as high as 0.020. The ghost of the back facet lines can be seen in a proper direction, with strong polychromatic properties, and linear gas-liquid often develops inside.
Green fluorite is often blue in tone, similar to turquoise emerald, but its surface luster is much weaker, because the refractive index of fluorite is only 1.434, and fluorite is isotropic, with no polychromatic color, and its interior often shows abnormal extinction due to the development of four groups of cleavage. There are also two-phase or three-phase gas-liquid inclusions in banded development, but the boundaries of gas-liquid inclusions are not as clear as those in emeralds. The density is higher than that of emerald, which is 3. 18 (+0.07, -0. 18)g/cm3. Fluorite often has a strong fluorescence reaction under ultraviolet irradiation, and some even emit phosphorescence.
Green apatite apatite belongs to hexagonal system like emerald. Mohs hardness is 5, which is lower than emerald, but the refractive index and density are higher than emerald, which are about 1.634 ~ 1.638 and 3. 18g/cm3, respectively. The absorption double line can be seen at 580 nm.
Emerald translucent and almost transparent arc jadeite is very similar to arc emerald. By magnifying observation, we can distinguish the two, because emerald is a single crystal with gas-liquid inclusions and primary or secondary cracks, while jadeite is a polycrystalline aggregate, often showing a fiber interwoven structure. On the polished surface, you can observe the boundary of the crystal, or the faint concave and convex marks left by the different directions of the crystal during polishing. Moreover, the refractive index of jadeite is higher than that of emerald, about 1.66, and the value generally obtained by curve emerald point measurement method is 1.58. The density of jadeite is higher than that of emerald, about 3.34g/cm3. The absorption spectrum of jadeite jade has three evenly spaced absorption lines (690nm, 660nm and 630nm) in the red region and 437nm in the blue region, which is different from jadeite jade.
Glass-green glass is most similar to emerald, and can be confused in color and appearance. Bubbles or other residues in glass will produce an appearance similar to cracks or fingerprint inclusions. However, glass lacks the green velvet feeling of emeralds, but when some emeralds are light in color, their strength is not enough to distinguish them from glass. Special care should be taken in the evaluation. First of all, glass is isotropic, but jadeite is not. It can be directly distinguished by polarizer, dichroic mirror or refractometer. Under the polarizer, glass often has abnormal extinction and no dichroism. The refractive index of glass ranges from 1.47 to 1.70 or even higher, which is obviously different from emerald.
Artificial yttrium aluminum garnet Artificial yttrium aluminum garnet is isotropic, and its difference is similar to that of glass. However, the artificial yttrium aluminum garnet has a large refractive index, which is generally greater than 1.8 1, so it cannot be measured by refractometer. The interior of artificial yttrium aluminum garnet is generally clean, with occasional bubbles. The absorption spectrum is different from emerald. Artificial yttrium aluminum garnet has a red flash visible to the naked eye, but it does not have the soft feeling of emerald velvet.
Synthetic spinel The spinel synthesized by flame melting method is isotropic, with a refractive index of about 1.728, and speckle extinction often occurs under polarizer. There may be bubbles and arc growth lines inside.
There are many forms of inlaid gems, including two or three layers. The most common ones are: emerald and beryl, emerald or beryl and green glass, red garnet and green glass, colorless crystal and a layer of green substance and colorless crystal. The top is usually made of emerald or beryl, and the bottom is lined with various green materials such as glass, synthetic emerald and light beryl. , the middle can be glued with colorless or green glue.
(2) Identification of synthetic emeralds
At present, emerald is usually synthesized by flux growth method and hydrothermal method. Because their growth environment is similar to that of natural emeralds, their physical and chemical characteristics are basically similar, but some properties are still different: synthetic emeralds are rich in color, have strong red fluorescence, and appear bright red under Charles filter. The typical characteristics of inclusions are cloud-like opaque unmelted melt and flux inclusions, silvery white opaque triangular platinum inclusions and so on. However, natural emeralds have their unique characteristics of inclusions, which are obviously different from the origin.
(3) Optimize the identification of emeralds.
The main methods to optimize emeralds are soaking in oil, dyeing and painting.
Due to the development of emerald cracks, oil is often used to cover the cracks and enhance the green color. Attention should be paid to the crack extending to the surface of the gem during identification. When the gem is illuminated by a dome light under a magnifying glass, the crack will produce interference color. If the gemstone is heated under a desk lamp, oil beads will flow out and emit yellow fluorescence under ultraviolet light.
Dyeing treatment uses chemical pigments to dye light or colorless emeralds into dark green. This green pigment is generally distributed along the cracks, and the whole is cobweb-like. Yellow-green fluorescence under long-wave ultraviolet light. There may be an absorption band at 630 ~ 660 nm.
If it is found that emerald jewelry is inlaid with a fully enclosed metal bracket during foil lining treatment, it is doubtful whether the outer edge of the pavilion of the gem is coated with a green film.
In the coating process, colorless beryl is often used as the core, and emerald films are grown and synthesized on the outer layer. The coating is easy to crack and stagger, which is an important feature of coated emeralds. Because the main body is beryl, the typical beryl inclusion is also an important identification feature. In addition, because the color is mainly concentrated in the coating, this emerald will have obvious characteristics in the immersion liquid.
(4) Identification of emeralds from different places.
For emeralds, it is of great significance to identify different producing areas. The identification characteristics of emeralds in main producing areas are as follows.
Colombian emeralds are the highest quality emeralds in the world, and the emeralds produced are pure green, yellowish green or slightly blue-green. The typical producing areas are Muzo, Chivor and Gachala deposits. Muzo emerald has the most beautiful color, while Chivor emerald has fewer defects. Colombia emerald has many cracks, and the cracks are sometimes filled with brown iron film. There are typical gas, liquid and solid inclusions, as well as fibrous inclusions, yellow-brown granular bastnaesite inclusions, pyrite inclusions, timely inclusions, pyrrhotite inclusions and molybdenite inclusions. Under the Charles filter, Colombian emeralds are red, while emeralds in other places generally do not change color.
Russian emeralds are mostly yellow-green, slightly lighter than those produced in Colombia. There are few cracks, and the typical inclusions are actinolite, which looks like a bamboo tube, commonly known as bamboo-like inclusions, green and brown, with irregular arrangement. In addition, flaky biotite inclusions are also common, which is also the reason why emeralds are brown.
Brazilian emeralds are characterized by small size, many flaws and generally poor quality, but sometimes the quality is good.
Zimbabwean emeralds are mainly produced in Belingwe Nature Reserve and other areas, which are characterized by small volume and rare quality emeralds exceeding one third of carats.
Tanzanian emeralds are mainly produced on the south bank of Manyara Lake, similar to Muzo emeralds, usually yellow-green, and sometimes blue-green varieties similar to Chivor gems are found. The quality of this gem is sometimes comparable to that of Colombian grandmother.
The colors of Zambian emeralds vary from bright green, blue-green to dark green. However, all Zambian gems are slightly gray in color, and there may be inclusions such as mica, hornblende, actinolite or tremolite.
Indian emeralds have typical comma-like inclusions, which are composed of two-phase (liquid and gas) inclusions and mica crystals in holes.
Pakistani emeralds have mica flakes and two-phase inclusions.
aquamarine
Aquamarine is far less precious than emerald, but as the birthstone of March, aquamarine has long been loved by people and is a symbol of happiness and youth. The name aquamarine comes from Latin. In the Middle Ages, people thought it could give the wearer insight and foresight. Some people think it has hypnotic ability. They think that the population contains aquamarine, so he can summon the devil from hell and get the answer to any question he wants. They also believe that it has the power to suppress evil and enable the wearer to overcome evil.
(1) identification
The identification of aquamarine is relatively easy, mainly the difference from imitation and the identification of a small number of optimized products. Aquamarine is sky blue and light sky blue, and often contains typical rain-like inclusions, which is obviously different from other similar gems. On this basis, through other physical characteristics and system tests, it is easy to distinguish.
(2) Economic evaluation
The economic evaluation of aquamarine is based on color, transparency and weight. Usually, the ones with dark color, flawless interior and heavy weight are the best and have high economic value.
Three. Genesis and source of the deposit
1. Genesis of the deposit
Beryllium in beryl is mainly distributed in I-type granite, so in the past, people thought that the formation of beryl minerals was mainly directly or indirectly related to this kind of granite, and the actual situation was roughly the same. Up to now, it has been found that most beryl occurs in rocky granite, which is formed by gas and liquid filling cracks in surrounding rock in the late magmatic activity. Due to the sufficient crystallization time, the crystallized single crystal is very large, which means it is called pegmatite in geology, which means that the mineral crystals in it are very large. However, these huge crystals are wrapped in quartz, feldspar and so on. And they have become continuous fragments for various reasons, and there are few clean areas that can be used as gems. However, if there are air gaps or mineral pockets in pegmatite, beryl crystals formed in pegmatite can become transparent and have fewer defects, thus making it have gem quality.
There are countless granite distribution areas in the world, but beryl mineralization only exists in a few pegmatites in Brazil, Argentina, Afghanistan, Africa, India, Madagascar, China and the United States. At the same time, although thousands of tons of pegmatite beryl minerals have been mined from these metallogenic areas, few of them can be used as gems. Therefore, beryl gem is relatively rare, and it will be even rarer as the pegmatite near the surface is gradually mined. There are also beautiful beryl crystals (especially aquamarine and golden beryl) in the vein rock mass, which cannot be classified as pegmatite. Sometimes these veins will form along the cracks in the surrounding rocks, forming a rock mass called greisen. In Russia, Ukraine and Mongolia, beautiful and transparent gem-grade aquamarine and golden beryl are found from this rock mass.
As mentioned above, most beryl gems discovered so far are indeed related to pegmatite. However, the past mining practice has proved that emerald, the darling of beryl gems, is rarely related to pegmatite. Emerald crystals often occur in flat sedimentary rocks or flat lenticular ore bodies in the contact zone between iron-rich basic rocks and acid rocks. The heat and pressure in the contact zone transform the primary minerals into new mineral varieties during metamorphism, such as beryl existing in pegmatite and flat sedimentary rocks, and its components enter mica schist under metamorphism. When recrystallizing there, if there is still a trace of chromium or vanadium in the parent rock, beryl will form emeralds. That is to say, in schist emeralds, beryllium mineralization is provided by primary granite slurry on one side, and its coloring elements belong to basic rocks on the other side of the contact zone. Emerald formed by schist deposition can only be a small piece with poor shape, developed cracks and full of inclusions. The most important emerald deposit type is Columbia type, and its genesis is very special. Compared with other types of deposits, the formation temperature and pressure of Colombian emeralds are lower. This deposit was formed by calcite infiltrating into the fault zone filled with limestone and shale. This calcite is usually grayish white and contains pyrite, quartz, albite, emerald and other minerals. This mineralization seems to belong to some kind of granite slurry source, but it has not been confirmed so far. Therefore, it can be said that the genesis of this deposit has not been clarified. There are also green beryls from rhyolite, which were found in Utah and later in New Mexico, and beryls from dolomite marble combined with iron-rich ultrabasic rocks. In a word, the origin of beryl is still an unclear problem.
2. Country of origin
The highest quality emeralds in the world are produced in Colombia. It is generally believed that the emeralds in Muzo Mine are the best, followed by Chival and Koskell. In addition to Colombia, Urals Mountain, Saint Davenport in Zimbabwe, Rajasthan in India, Brazil, Zambia, Austria, Australia, South Africa, Tanzania, Norway, the United States and Pakistan also produce emeralds. In 1990s, we discovered an emerald deposit in Wenshan Prefecture, Yunnan Province, but its gemological value was not high.
The world high-quality aquamarine is mainly produced in Brazil, accounting for more than 70% of the world aquamarine production. Up to now, the world's largest aquamarine crystal, with the mass of 1 10.5kg, is produced in Brazil. Ural Mountain is also an important supplier of high-quality aquamarine. In addition, aquamarine is also produced in China, USA, Myanmar, South Africa, Zimbabwe and India. Except emeralds and aquamarine, other beryls are distributed in many countries in the world, but the main producing areas are Brazil, Madagascar, Namibia, the United States, China and so on.
Beryl gem is a kind of gem with mineral beryl as raw material. Due to different geological causes, they contain different chromogenic elements and show different colors, thus forming different kinds of gems. Among them, the chromogenic element Cr is emerald, which is a world-famous precious gem emerald. The related knowledge in this chapter needs to be comprehensively and systematically studied and mastered. The basic properties of beryl must be remembered. We should firmly grasp the characteristics of emeralds and the knowledge of appraisal and evaluation. The identification and evaluation of aquamarine also need to be fully understood. In addition, it is also necessary to know the origin and origin of beryl gems, especially the particularity of the origin of the emerald deposit in Colombia.
Practice and thinking
1. What is beryl gem? What are the main varieties? What is the classification basis of gem types?
2. Describe the chemical composition and physical properties of beryl?
3. How to determine emeralds from different places of origin?
4. How to distinguish natural emeralds from synthetic emeralds?
5. How to distinguish natural emeralds from treated emeralds?
6. Briefly describe the quality evaluation basis of emerald.
7. Briefly describe the identification and quality evaluation methods of aquamarine.