Emerald is a gemstone and a variety of the mineral beryl colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5–8 on the Mohs scale. Most emeralds are included, so their toughness is classified as poor. Emerald is a cyclosilicate; the word "emerald" is derived, from Vulgar Latin: esmaralda/esmaraldus, a variant of Latin smaragdus, which originated in Ancient Greek: σμάραγδος. Emeralds, like all colored gemstones, are graded using four basic parameters–the four Cs of connoisseurship: color, clarity and carat weight. In the grading of colored gemstones, color is by far the most important criterion. However, in the grading of emeralds, clarity is considered a close second. A fine emerald must possess not only a pure verdant green hue as described below, but a high degree of transparency to be considered a top gem. In the 1960s, the American jewelry industry changed the definition of emerald to include the green vanadium-bearing beryl; as a result, vanadium emeralds purchased as emeralds in the United States are not recognized as such in the UK and Europe.
In America, the distinction between traditional emeralds and the new vanadium kind is reflected in the use of terms such as "Colombian emerald". In gemology, color is divided into three components: hue and tone. Emeralds occur in hues ranging from yellow-green to blue-green, with the primary hue being green. Yellow and blue are the normal secondary hues found in emeralds. Only gems that are medium to dark in tone are considered emeralds; the finest emeralds are 75% tone on a scale where 0% tone is colorless and 100% is opaque black. In addition, a fine emerald will be saturated and have a hue, bright. Gray is the normal saturation mask found in emeralds. Emeralds tend to surface breaking fissures. Unlike diamonds, where the loupe standard, i.e. 10× magnification, is used to grade clarity, emeralds are graded by eye. Thus, if an emerald has no visible inclusions to the eye it is considered flawless. Stones that lack surface breaking fissures are rare and therefore all emeralds are treated to enhance the apparent clarity.
The inclusions and fissures within an emerald are sometime described as jardin, because of their mossy appearance. Imperfections can be used to identify a particular stone. Eye-clean stones of a vivid primary green hue, with no more than 15% of any secondary hue or combination of a medium-dark tone, command the highest prices; the relative non-uniformity motivates the cutting of emeralds in cabochon form, rather than faceted shapes. Faceted emeralds are most given an oval cut, or the signature emerald cut, a rectangular cut with facets around the top edge. Most emeralds are oiled as part of the post-lapidary process, in order to fill in surface-reaching cracks so that clarity and stability are improved. Cedar oil, having a similar refractive index, is used in this adopted practice. Other liquids, including synthetic oils and polymers with refractive indexes close to that of emeralds, such as Opticon, are used; these treatments are applied in a vacuum chamber under mild heat, to open the pores of the stone and allow the fracture-filling agent to be absorbed more effectively.
The U. S. Federal Trade Commission requires the disclosure of this treatment when an oil treated emerald is sold; the use of oil is traditional and accepted by the gem trade, although oil treated emeralds are worth much less than un-treated emeralds of similar quality. Other treatments, for example the use of green-tinted oil, are not acceptable in the trade. Gems are graded on a four-step scale; these categories reflect levels of enhancement, not clarity. A gem graded. Laboratories apply these criteria differently; some gemologists consider the mere presence of oil or polymers to constitute enhancement. Others may ignore traces of oil if the presence of the material does not improve the look of the gemstone. Emeralds in antiquity were mined in Egypt at locations on Mount Smaragdus since 1500 BCE, India, Austria since at least the 14th century CE; the Egyptian mines were exploited on an industrial scale by the Roman and Byzantine Empires, by Islamic conquerors. Mining ceased with the discovery of the Colombian deposits.
Colombia is by far the world's largest producer of emeralds, constituting 50–95% of the world production, with the number depending on the year and grade. Emerald production in Colombia has increased drastically in the last decade, increasing by 78% from 2000 to 2010; the three main emerald mining areas in Colombia are Muzo and Chivor. Rare "trapiche" emeralds are found in Colombia, distinguished by ray-like spokes of dark impurities. Zambia is the world's second biggest producer, with its Kafubu River area deposits about 45 km southwest of Kitwe responsible for 20% of the world's production of gem-quality stones in 2004. In the first half of 2011, the Kagem Mines produced 3.74 tons of emeralds. Emeralds are found all over the world in countries such as Afghanistan, Austria, Bulgaria, Canada, Egypt, France, India, Kazakhstan, Mozambique, Nigeria, Pakistan
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum. Other varieties of gem-quality corundum are called sapphires. Ruby is one of the traditional cardinal gems, together with amethyst, sapphire and diamond; the word ruby comes from Latin for red. The color of a ruby is due to the element chromium; some gemstones that are popularly or called rubies, such as the Black Prince's Ruby in the British Imperial State Crown, are spinels. These were once known as "Balas rubies"; the quality of a ruby is determined by its color and clarity, along with carat weight, affect its value. The brightest and most valuable shade of red called blood-red or pigeon blood, commands a large premium over other rubies of similar quality. After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions may indicate that the stone has been treated. Ruby is the traditional birthstone for July and is pinker than garnet, although some rhodolite garnets have a similar pinkish hue to most rubies.
The world's most valuable ruby is the Sunrise Ruby. Rubies have a hardness of 9.0 on the Mohs scale of mineral hardness. Among the natural gems only moissanite and diamond are harder, with diamond having a Mohs hardness of 10.0 and moissanite falling somewhere in between corundum and diamond in hardness. Sapphire and pure corundum are α-alumina, the most stable form of Al2O3, in which 3 electrons leave each aluminum ion to join the regular octahedral group of six nearby O2− ions; when a chromium atom replaces an occasional aluminum atom, it too loses 3 electrons to become a chromium3+ ion to maintain the charge balance of the Al2O3 crystal. However, the Cr3 + ions have electron orbitals in different directions than aluminum; the octahedral arrangement of the O2− ions is distorted, the energy levels of the different orbitals of those Cr3+ ions are altered because of the directions to the O2− ions. Those energy differences correspond to absorption in the ultraviolet and yellow-green regions of the spectrum.
If one percent of the aluminum ions are replaced by chromium in ruby, the yellow-green absorption results in a red color for the gem. Additionally, absorption at any of the above wavelengths stimulates fluorescent emission of 694-nanometer-wavelength red light, which adds to its red color and perceived luster. After absorbing short-wavelength light, there is a short interval of time when the crystal lattice of ruby is in an excited state before fluorescence occurs. If 694-nanometer photons pass through the crystal during that time, they can stimulate more fluorescent photons to be emitted in-phase with them, thus strengthening the intensity of that red light. By arranging mirrors or other means to pass emitted light through the crystal, a ruby laser in this way produces a high intensity of coherent red light. All natural rubies have imperfections in them, including color impurities and inclusions of rutile needles known as "silk". Gemologists use these needle inclusions found in natural rubies to distinguish them from synthetics, simulants, or substitutes.
The rough stone is heated before cutting. These days all rubies are treated in some form, with heat treatment being the most common practice. Untreated rubies of high quality command a large premium; some rubies show a three-point or six-point asterism or "star". These rubies are cut into cabochons to display the effect properly. Asterisms are best visible with a single-light source and move across the stone as the light moves or the stone is rotated; such effects occur. This is one example. Furthermore, rubies can show color changes—though this occurs rarely—as well as chatoyancy or the "cat's eye" effect. Gemstone-quality corundum in all shades of red, including pink, are called rubies. However, in the United States, a minimum color saturation must be met to be called a ruby. Drawing a distinction between rubies and pink sapphires is new, having arisen sometime in the 20th century; the distinction between ruby and pink sapphire is not clear and can be debated. As a result of the difficulty and subjectiveness of such distinctions, trade organizations such as the International Colored Gemstone Association have adopted the broader definition for ruby which encompasses its lighter shades, including pink.
The Mogok Valley in Upper Myanmar was for centuries the world's main source for rubies. That region has produced some exceptional rubies, however in recent years few good rubies have been found. In central Myanmar, the area of Mong Hsu began producing rubies during the 1990s and became the world's main ruby mining area; the most found ruby deposit in Myanmar is in Namya located in the northern state of Kachin. Rubies have been mined in Thailand, in the Pailin and Samlout District of Cambodia, as well as in Afghanistan, Brazil, India, Namibia and Scotland. In Sri Lanka, lighter shades of rubies are more found; the Republic of Macedonia is the only country in mainland Europe to have occurring rubies. They can be found around the city of
The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula BeAl2O4. The name chrysoberyl is derived from the Greek words χρυσός chrysos and βήρυλλος beryllos, meaning "a gold-white spar". Despite the similarity of their names and beryl are two different gemstones, although they both contain beryllium. Chrysoberyl is the third-hardest encountered natural gemstone and lies at 8.5 on the Mohs scale of mineral hardness, between corundum and topaz. An interesting feature of its crystals are the cyclic twins called trillings; these twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each "twin" oriented at 120° to its neighbors and taking up 120° of the cyclic trilling. If only two of the three possible twin orientations are present, a "V"-shaped twin results. Ordinary chrysoberyl is transparent to translucent; when the mineral exhibits good pale green to yellow color and is transparent it is used as a gemstone. The three main varieties of chrysoberyl are: ordinary yellow-to-green chrysoberyl, cat's eye or cymophane, alexandrite.
Yellow-green chrysoberyl was referred to as "chrysolite" during the Victorian and Edwardian eras, which caused confusion since that name has been used for the mineral olivine. Alexandrite, a pleochroic gem, will exhibit emerald green and orange-yellow colors depending on viewing direction in polarised light. However, its most distinctive property is that it changes color in artificial light compared to daylight; the color change from red to green is due to strong absorption of light in a narrow yellow portion of the spectrum, while allowing large bands of more blue-green and red wavelengths to be transmitted. Which of these prevails to give the perceived hue depends on the spectral balance of the illumination. Fine-quality alexandrite has a green to bluish-green color in daylight, changing to a red to purplish-red color in incandescent light. However, fine-color material is rare. Less-desirable stones may have daylight colors of yellowish-green and incandescent colors of brownish red. Cymophane is popularly known as "cat's eye".
This variety exhibits pleasing opalescence that reminds one of the eye of a cat. When cut to produce a cabochon, the mineral forms a light-green specimen with a silky band of light extending across the surface of the stone. Chrysoberyl forms as a result of pegmatitic processes. Melting in the Earth's crust produces low-density molten magma which can rise upwards towards the surface; as the main magma body cools, water present in low concentrations became more concentrated in the molten rock because it could not be incorporated into the crystallization of solid minerals. The remnant magma thus becomes richer in water, in rare elements that do not fit in the crystal structures of major rock-forming minerals; the water extends the temperature range downwards before the magma becomes solid, allowing concentration of rare elements to proceed so far that they produce their own distinctive minerals. The resulting rock is igneous in appearance but formed at a low temperature from a water-rich melt, with large crystals of the common minerals such as quartz and feldspar, but with elevated concentrations of rare elements such as beryllium, lithium, or niobium forming their own minerals.
The high water content of the magma made it possible for the crystals to grow so pegmatite crystals are quite large, which increases the likelihood of gem specimens forming. Chrysoberyl can grow in the country rocks near to pegmatites, when Be- and Al-rich fluids from the pegmatite react with surrounding minerals. Hence, it can be found in contact with metamorphic deposits of dolomitic marble; because it is a hard, dense mineral, resistant to chemical alteration, it can be weathered out of rocks and deposited in river sands and gravels in alluvial deposits with other gem minerals such as diamond, topaz, spinel and tourmaline. When found in such placers, it will have rounded edges instead of wedge-shape forms. Much of the chrysoberyl mined in Brazil and Sri Lanka is recovered from placers, as the host rocks have been intensely weathered and eroded. If the pegmatite fluid is rich in beryllium, crystals of beryl or chrysoberyl could form. Beryl has a high ratio of beryllium to aluminium. Both are stable with the common mineral quartz.
For alexandrite to form, some chromium would have had to be present. However and chromium do not tend to occur in the same types of rock. Chromium is commonest in mafic and ultramafic rocks in which beryllium is rare. Beryllium becomes concentrated in felsic pegmatites in which chromium is absent. Therefore, the only situation where an alexandrite can grow is when Be-rich pegmatitic fluids react with Cr-rich country rock; this unusual requirement explains the rarity of this chrysoberyl variety. The alexandrite variety displays a color change dependent upon the nature of ambient lighting. Alexandrite effect is the phenomenon of an observed color change from greenish to reddish with a change in source illumination. Alexandrite results from small scale replacement of aluminium by chromium ions in the crystal structure, which causes intense absorption of light over a narrow range of wavelengths in the yellow region of the visible light spectrum; because human vision is most s
Perestroika was a political movement for reformation within the Communist Party of the Soviet Union during the 1980s and 1990s and is associated with Soviet leader Mikhail Gorbachev and his glasnost policy reform. The literal meaning of perestroika is "restructuring", referring to the restructuring of the Soviet political and economic system. Perestroika is sometimes argued to be a significant cause of the dissolution of the Soviet Union, the revolutions of 1989 in Eastern Europe, the end of the Cold War. Perestroika allowed more independent actions from various ministries and introduced some market-like reforms; the goal of perestroika, was not to end the command economy but rather to make socialism work more efficiently to better meet the needs of Soviet citizens. The process of implementing perestroika arguably exacerbated existing political and economic tensions within the Soviet Union and is blamed for furthering the political ascent of nationalism and nationalist political parties in the constituent republics.
Perestroika and its associated structural ailments have been cited as major catalysts leading to the dissolution of the Soviet Union. In May 1985, Gorbachev gave a speech in Leningrad in which he admitted the slowing of economic development, inadequate living standards; this was the first time. The program was furthered at the 27th Congress of the Communist Party in Gorbachev's report to the congress, in which he spoke about "perestroika", "uskoreniye", "human factor", "glasnost", "expansion of the khozraschyot". During the initial period of Mikhail Gorbachev's time in power, he talked about modifying central planning but did not make any fundamental changes. Gorbachev and his team of economic advisors introduced more fundamental reforms, which became known as perestroika. At the June 1987 plenary session of the Central Committee of the Communist Party of the Soviet Union, Gorbachev presented his "basic theses", which laid the political foundation of economic reform for the remainder of the existence of the Soviet Union.
In July 1987, the Supreme Soviet of the Soviet Union passed the Law on State Enterprise. The law stipulated that state enterprises were free to determine output levels based on demand from consumers and other enterprises. Enterprises had to fulfill state orders, but they could dispose of the remaining output as they saw fit. However, at the same time the state still held control over the means of production for these enterprises, thus limiting their ability to enact full-cost accountability. Enterprises bought input from suppliers at negotiated contract prices. Under the law, enterprises became self-financing. No longer was the government to rescue unprofitable enterprises; the law shifted control over the enterprise operations from ministries to elected workers' collectives. Gosplan's responsibilities were to supply general guidelines and national investment priorities, not to formulate detailed production plans; the Law on Cooperatives, enacted in May 1988, was the most radical of the economic reforms during the early part of the Gorbachev era.
For the first time since Vladimir Lenin's New Economic Policy was abolished in 1928, the law permitted private ownership of businesses in the services and foreign-trade sectors. The law imposed high taxes and employment restrictions, but it revised these to avoid discouraging private-sector activity. Under this provision, cooperative restaurants and manufacturers became part of the Soviet scene. Gorbachev brought perestroika to the Soviet Union's foreign economic sector with measures that Soviet economists considered bold at that time, his programme eliminated the monopoly that the Ministry of Foreign Trade had once held on most trade operations. It permitted the ministries of the various industrial and agricultural branches to conduct foreign trade in sectors under their responsibility, rather than having to operate indirectly through the bureaucracy of trade ministry organizations. In addition and local organizations and individual state enterprises were permitted to conduct foreign trade.
This change was an attempt to redress a major imperfection in the Soviet foreign trade regime: the lack of contact between Soviet end users and suppliers and their foreign partners. The most significant of Gorbachev's reforms in the foreign economic sector allowed foreigners to invest in the Soviet Union in the form of joint ventures with Soviet ministries, state enterprises, cooperatives; the original version of the Soviet Joint Venture Law, which went into effect in June 1987, limited foreign shares of a Soviet venture to 49 percent and required that Soviet citizens occupy the positions of chairman and general manager. After potential Western partners complained, the government revised the regulations to allow majority foreign ownership and control. Under the terms of the Joint Venture Law, the Soviet partner supplied labor, a large domestic market; the foreign partner supplied capital, entrepreneurial expertise, in many cases and services of world competitive quality. Gorbachev's economic changes did not do much to restart the country's sluggish economy in the late 1980s.
The reforms decentralised things to some extent, although price controls remained, as did the ruble's inconvertibility and
Bangkok is the capital and most populous city of Thailand. It is known in Thai as Krung Thep Maha Nakhon or Krung Thep; the city occupies 1,568.7 square kilometres in the Chao Phraya River delta in central Thailand, has a population of over eight million, or 12.6 percent of the country's population. Over fourteen million people lived within the surrounding Bangkok Metropolitan Region at the 2010 census, making Bangkok the nation's primate city dwarfing Thailand's other urban centres in terms of importance. Bangkok traces its roots to a small trading post during the Ayutthaya Kingdom in the 15th century, which grew and became the site of two capital cities: Thonburi in 1768 and Rattanakosin in 1782. Bangkok was at the heart of the modernization of Siam renamed Thailand, during the late-19th century, as the country faced pressures from the West; the city was at the centre of Thailand's political struggles throughout the 20th century, as the country abolished absolute monarchy, adopted constitutional rule, underwent numerous coups and several uprisings.
The city grew during the 1960s through the 1980s and now exerts a significant impact on Thailand's politics, education and modern society. The Asian investment boom in the 1980s and 1990s led many multinational corporations to locate their regional headquarters in Bangkok; the city is now a regional force in business. It is an international hub for transport and health care, has emerged as a centre for the arts and entertainment; the city is known for cultural landmarks, as well as its red-light districts. The Grand Palace and Buddhist temples including Wat Arun and Wat Pho stand in contrast with other tourist attractions such as the nightlife scenes of Khaosan Road and Patpong. Bangkok is among the world's top tourist destinations, has been named the world's most visited city in several rankings. Bangkok's rapid growth coupled with little urban planning has resulted in a haphazard cityscape and inadequate infrastructure. An inadequate road network, despite an extensive expressway network, together with substantial private car usage, have led to chronic and crippling traffic congestion, which caused severe air pollution in the 1990s.
The city has since turned to public transport in an attempt to solve the problem. Five rapid transit lines are now in operation, with more systems under construction or planned by the national government and the Bangkok Metropolitan Administration; the history of Bangkok dates at least back to the early 15th century, when it was a village on the west bank of the Chao Phraya River, under the rule of Ayutthaya. Because of its strategic location near the mouth of the river, the town increased in importance. Bangkok served as a customs outpost with forts on both sides of the river, was the site of a siege in 1688 in which the French were expelled from Siam. After the fall of Ayutthaya to the Burmese Empire in 1767, the newly crowned King Taksin established his capital at the town, which became the base of the Thonburi Kingdom. In 1782, King Phutthayotfa Chulalok succeeded Taksin, moved the capital to the eastern bank's Rattanakosin Island, thus founding the Rattanakosin Kingdom; the City Pillar was erected on 21 April 1782, regarded as the date of foundation of the present city.
Bangkok's economy expanded through international trade, first with China with Western merchants returning in the early to-mid 19th century. As the capital, Bangkok was the centre of Siam's modernization as it faced pressure from Western powers in the late-19th century; the reigns of Kings Mongkut and Chulalongkorn saw the introduction of the steam engine, printing press, rail transport and utilities infrastructure in the city, as well as formal education and healthcare. Bangkok became the centre stage for power struggles between the military and political elite as the country abolished absolute monarchy in 1932. Allied with Japan in World War II, it was subjected to Allied bombing, but grew in the post-war period as a result of US aid and government-sponsored investment. Bangkok's role as a US military R&R destination boosted its tourism industry as well as establishing it as a sex tourism destination. Disproportionate urban development led to increasing income inequalities and migration from rural areas into Bangkok.
Following the US withdrawal from Vietnam in 1973, Japanese businesses took over as leaders in investment, the expansion of export-oriented manufacturing led to growth of the financial market in Bangkok. Rapid growth of the city continued through the 1980s and early 1990s, until it was stalled by the 1997 Asian financial crisis. By many public and social issues had emerged, among them the strain on infrastructure reflected in the city's notorious traffic jams. Bangkok's role as the nation's political stage continues to be seen in strings of popular protests, from the student uprisings in 1973 and 1976, anti-military demonstrations in 1992, successive anti-government demonstrations by opposing groups from 2008 on. Administration of the city was first formalized by King Chulalongkorn in 1906, with the establishment of Monthon Krung Thep Phra Maha Nakhon as a national subdivision. In 1915 the monthon was split into several provinces, the administrative boundaries of which have since further changed.
The city in its current form was created in 1972 with the formation of the Bangkok Metropolitan Administration, following the merger of Phra Nakhon Province on the eastern bank of the Chao Phraya and Thonburi Province on the west during the previous year. The origin of th
Beryl is a mineral composed of beryllium aluminium cyclosilicate with the chemical formula Be3Al2Si6O18. Well-known varieties of beryl include aquamarine. Occurring, hexagonal crystals of beryl can be up to several meters in size, but terminated crystals are rare. Pure beryl is colorless, but it is tinted by impurities. Beryl is an ore source of beryllium; the name "beryl" is derived from Greek βήρυλλος beryllos which referred to a "precious blue-green color-of-sea-water stone". The term was adopted for the mineral beryl more exclusively; when the first eyeglasses were constructed in 13th century Italy, the lenses were made of beryl as glass could not be made clear enough. Glasses were named Brillen in German. Beryl of various colors is found most in granitic pegmatites, but occurs in mica schists in the Ural Mountains, limestone in Colombia. Beryl is associated with tin and tungsten ore bodies. Beryl is found in Europe in Norway, Germany, Sweden and Russia, as well as Brazil, Madagascar, Pakistan, South Africa, the United States, Zambia.
US beryl locations are in California, Connecticut, Idaho, New Hampshire, North Carolina, South Dakota and Utah. New England's pegmatites have produced some of the largest beryls found, including one massive crystal from the Bumpus Quarry in Albany, Maine with dimensions 5.5 by 1.2 m with a mass of around 18 metric tons. As of 1999, the world's largest known occurring crystal of any mineral is a crystal of beryl from Malakialina, Madagascar, 18 m long and 3.5 m in diameter, weighing 380,000 kg. Beryl belongs to the hexagonal crystal system. Beryl forms hexagonal columns but can occur in massive habits; as a cyclosilicate beryl incorporates rings of silicate tetrahedra of Si 6 O 18 that are arranged in columns along the C axis and as parallel layers perpendicular to the C axis, forming channels along the C axis. These channels permit a variety of ions, neutral atoms, molecules to be incorporated into the crystal thus disrupting the overall charge of the crystal permitting further substitutions in Aluminium and Beryllium sites in the crystal structure.
These impurities give rise to the variety of colors of beryl. Increasing alkali content within the silicate ring channels causes increases to the refractive indices and birefringence. Aquamarine is a cyan variety of beryl, it occurs at most localities. The gem-gravel placer deposits of Sri Lanka contain aquamarine. Green-yellow beryl, such as that occurring in Brazil, is sometimes called chrysolite aquamarine; the deep blue version of aquamarine is called maxixe. Maxixe is found in the country of Madagascar, its color fades to white when exposed to sunlight or is subjected to heat treatment, though the color returns with irradiation. The pale blue color of aquamarine is attributed to Fe2+. Fe3+ ions produce golden-yellow color, when both Fe2+ and Fe3+ are present, the color is a darker blue as in maxixe. Decoloration of maxixe by light or heat thus may be due to the charge transfer between Fe3+ and Fe2+. Dark-blue maxixe color can be produced in green, pink or yellow beryl by irradiating it with high-energy particles.
In the United States, aquamarines can be found at the summit of Mt. Antero in the Sawatch Range in central Colorado. In Wyoming, aquamarine has been discovered near Powder River Pass. Another location within the United States is the Sawtooth Range near Stanley, although the minerals are within a wilderness area which prevents collecting. In Brazil, there are mines in the states of Minas Gerais, Espírito Santo, Bahia, minorly in Rio Grande do Norte; the mines of Colombia, Madagascar, Malawi and Kenya produce aquamarine. The largest aquamarine of gemstone quality mined was found in Marambaia, Minas Gerais, Brazil, in 1910, it weighed over 110 kg, its dimensions were 48.5 cm long and 42 cm in diameter. The largest cut aquamarine gem is the Dom Pedro aquamarine, now housed in the Smithsonian Institution's National Museum of Natural History; the ancient Romans believed that aquamarine would protect against any dangers while travelling at sea, that it provided energy and cured laziness. Emerald is green beryl, sometimes vanadium.
Most emeralds are included, so their brittleness is classified as poor. The modern English word "emerald" comes via Middle English Emeraude, imported from modern French via Old French Ésmeraude and Medieval Latin Esmaraldus, from Latin smaragdus, from Greek σμάραγδος smaragdos meaning ‘green gem’, from Hebrew ברקת bareket, meaning ‘lightning flash’, referring to ‘emerald’, relating to Akkadian baraqtu, meaning ‘emerald’, relating to the Sanskrit word मरकत marakata, meaning ‘green’; the Semitic word אזמרגד izmargad, meaning ` emerald', is a back-loan. Emeralds in antiquity were mined by the Egyptians and in what is now Austria, as well as Swat in contemporary Pakistan. A ra
A gemstone is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks and organic materials that are not minerals are used for jewelry and are therefore considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity is another characteristic. Apart from jewelry, from earliest antiquity engraved gems and hardstone carvings, such as cups, were major luxury art forms. A gem maker is called a gemcutter; the traditional classification in the West, which goes back to the ancient Greeks, begins with a distinction between precious and semi-precious. In modern use the precious stones are diamond, ruby and emerald, with all other gemstones being semi-precious; this distinction reflects the rarity of the respective stones in ancient times, as well as their quality: all are translucent with fine color in their purest forms, except for the colorless diamond, hard, with hardnesses of 8 to 10 on the Mohs scale.
Other stones are classified by their color and hardness. The traditional distinction does not reflect modern values, for example, while garnets are inexpensive, a green garnet called tsavorite can be far more valuable than a mid-quality emerald. Another unscientific term for semi-precious gemstones used in art history and archaeology is hardstone. Use of the terms'precious' and'semi-precious' in a commercial context is, misleading in that it deceptively implies certain stones are intrinsically more valuable than others, not the case. In modern times gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology; the first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon and rubies of aluminium oxide. Next, many gems are crystals which are classified by their crystal system such as cubic or trigonal or monoclinic. Another term used is habit, the form the gem is found in.
For example, which have a cubic crystal system, are found as octahedrons. Gemstones are classified into different groups and varieties. For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Other examples are the emerald, red beryl, goshenite and morganite, which are all varieties of the mineral species beryl. Gems are characterized in terms of refractive index, specific gravity, cleavage and luster, they may exhibit double refraction. They may have a distinctive absorption spectrum. Material or flaws within a stone may be present as inclusions. Gemstones may be classified in terms of their "water"; this is a recognized grading of the gem's luster, transparency, or "brilliance". Transparent gems are considered "first water", while "second" or "third water" gems are those of a lesser transparency. There is no universally accepted grading system for gemstones. Diamonds are graded using a system developed by the Gemological Institute of America in the early 1950s.
All gemstones were graded using the naked eye. The GIA system included a major innovation: the introduction of 10x magnification as the standard for grading clarity. Other gemstones are still graded using the naked eye. A mnemonic device, the "four Cs", has been introduced to help the consumer understand the factors used to grade a diamond. With modification, these categories can be useful in understanding the grading of all gemstones; the four criteria carry different weight depending upon whether they are applied to colored gemstones or to colorless diamonds. In diamonds, cut is the primary determinant of value, followed by color. Diamonds are meant to sparkle, to break down light into its constituent rainbow colors, chop it up into bright little pieces, deliver it to the eye. In its rough crystalline form, a diamond will do none of these things. In gemstones that have color, including colored diamonds, it is the purity and beauty of that color, the primary determinant of quality. Physical characteristics that make a colored stone valuable are color, clarity to a lesser extent, unusual optical phenomena within the stone such as color zoning and asteria.
The Greeks, for example valued asteria gemstones, which were regarded as powerful love charms, Helen of Troy was known to have worn star-corundum. Aside from the diamond, the ruby, emerald and opal have been considered to be precious. Up to the discoveries of bulk amethyst in Brazil in the 19th century, amethyst was considered a precious stone as well, going back to ancient Greece. In the last century certain stones such as aquamarine and cat's eye have been popular and hence been regarded as precious. Today such a distinction is no longer made by the gemstone trade. Many gemstones are used in the most expensive jewelr