Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic material, a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees, or it is defined more broadly to include the same type of tissue elsewhere such as in the roots of trees or shrubs. In a living tree it performs a support function, enabling woody plants to grow large or to stand up by themselves, it conveys water and nutrients between the leaves, other growing tissues, the roots. Wood may refer to other plant materials with comparable properties, to material engineered from wood, or wood chips or fiber. Wood has been used for thousands of years for fuel, as a construction material, for making tools and weapons and paper. More it emerged as a feedstock for the production of purified cellulose and its derivatives, such as cellophane and cellulose acetate.
As of 2005, the growing stock of forests worldwide was about 434 billion cubic meters, 47% of, commercial. As an abundant, carbon-neutral renewable resource, woody materials have been of intense interest as a source of renewable energy. In 1991 3.5 billion cubic meters of wood were harvested. Dominant uses were for building construction. A 2011 discovery in the Canadian province of New Brunswick yielded the earliest known plants to have grown wood 395 to 400 million years ago. Wood can be dated by carbon dating and in some species by dendrochronology to determine when a wooden object was created. People have used wood for thousands of years for many purposes, including as a fuel or as a construction material for making houses, weapons, packaging and paper. Known constructions using wood date back ten thousand years. Buildings like the European Neolithic long house were made of wood. Recent use of wood has been enhanced by the addition of bronze into construction; the year-to-year variation in tree-ring widths and isotopic abundances gives clues to the prevailing climate at the time a tree was cut.
Wood, in the strict sense, is yielded by trees, which increase in diameter by the formation, between the existing wood and the inner bark, of new woody layers which envelop the entire stem, living branches, roots. This process is known as secondary growth; these cells go on to form thickened secondary cell walls, composed of cellulose and lignin. Where the differences between the four seasons are distinct, e.g. New Zealand, growth can occur in a discrete annual or seasonal pattern, leading to growth rings. If the distinctiveness between seasons is annual, these growth rings are referred to as annual rings. Where there is little seasonal difference growth rings are to be indistinct or absent. If the bark of the tree has been removed in a particular area, the rings will be deformed as the plant overgrows the scar. If there are differences within a growth ring the part of a growth ring nearest the center of the tree, formed early in the growing season when growth is rapid, is composed of wider elements.
It is lighter in color than that near the outer portion of the ring, is known as earlywood or springwood. The outer portion formed in the season is known as the latewood or summerwood. However, there are major differences, depending on the kind of wood; as a tree grows, lower branches die, their bases may become overgrown and enclosed by subsequent layers of trunk wood, forming a type of imperfection known as a knot. The dead branch may not be attached to the trunk wood except at its base, can drop out after the tree has been sawn into boards. Knots affect the technical properties of the wood reducing the local strength and increasing the tendency for splitting along the wood grain, but may be exploited for visual effect. In a longitudinally sawn plank, a knot will appear as a circular "solid" piece of wood around which the grain of the rest of the wood "flows". Within a knot, the direction of the wood is up to 90 degrees different from the grain direction of the regular wood. In the tree a knot is either the base of a dormant bud.
A knot is conical in shape with the inner tip at the point in stem diameter at which the plant's vascular cambium was located when the branch formed as a bud. In grading lumber and structural timber, knots are classified according to their form, size and the firmness with which they are held in place; this firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow. Knots materially affect cracking and warping, ease in working, cleavability of timber, they are defects which weaken timber and lower its value for structural purposes where strength is an important consideration. The weakening effect is much more serious when timber is subjected to forces perpendicular to the grain and/or tension than when under load along the grain and/or compression; the extent to which knots affect the strength of a beam depends upon their position, size and condition. A knot on the upper side is compressed. If there is a season check
Ireland is an island in the North Atlantic. It is separated from Great Britain to its east by the North Channel, the Irish Sea, St George's Channel. Ireland is the second-largest island of the British Isles, the third-largest in Europe, the twentieth-largest on Earth. Politically, Ireland is divided between the Republic of Ireland, which covers five-sixths of the island, Northern Ireland, part of the United Kingdom. In 2011, the population of Ireland was about 6.6 million, ranking it the second-most populous island in Europe after Great Britain. Just under 4.8 million live in the Republic of Ireland and just over 1.8 million live in Northern Ireland. The island's geography comprises low-lying mountains surrounding a central plain, with several navigable rivers extending inland, its lush vegetation is a product of its mild but changeable climate, free of extremes in temperature. Much of Ireland was woodland until the end of the Middle Ages. Today, woodland makes up about 10% of the island, compared with a European average of over 33%, most of it is non-native conifer plantations.
There are twenty-six extant mammal species native to Ireland. The Irish climate is influenced by the Atlantic Ocean and thus moderate, winters are milder than expected for such a northerly area, although summers are cooler than those in continental Europe. Rainfall and cloud cover are abundant; the earliest evidence of human presence in Ireland is dated at 10,500 BC. Gaelic Ireland had emerged by the 1st century AD; the island was Christianised from the 5th century onward. Following the 12th century Norman invasion, England claimed sovereignty. However, English rule did not extend over the whole island until the 16th–17th century Tudor conquest, which led to colonisation by settlers from Britain. In the 1690s, a system of Protestant English rule was designed to materially disadvantage the Catholic majority and Protestant dissenters, was extended during the 18th century. With the Acts of Union in 1801, Ireland became a part of the United Kingdom. A war of independence in the early 20th century was followed by the partition of the island, creating the Irish Free State, which became sovereign over the following decades, Northern Ireland, which remained a part of the United Kingdom.
Northern Ireland saw much civil unrest from the late 1960s until the 1990s. This subsided following a political agreement in 1998. In 1973 the Republic of Ireland joined the European Economic Community while the United Kingdom, Northern Ireland, as part of it, did the same. Irish culture has had a significant influence on other cultures in the field of literature. Alongside mainstream Western culture, a strong indigenous culture exists, as expressed through Gaelic games, Irish music and the Irish language; the island's culture shares many features with that of Great Britain, including the English language, sports such as association football, horse racing, golf. The names Éire derive from Old Irish Eriu; this in turn comes from the Proto-Celtic *Iveriu, the source of Latin Hibernia. Iveriu derives from a root meaning'fat, prosperous'. During the last glacial period, up until about 10,000 BC, most of Ireland was periodically covered in ice. Sea levels were lower and Ireland, like Great Britain, formed part of continental Europe.
By 16,000 BC, rising sea levels due to ice melting caused Ireland to become separated from Great Britain. Around 6000 BC, Great Britain itself became separated from continental Europe; the earliest evidence of human presence in Ireland is dated at 10,500 BC, demonstrated by a butchered bear bone found in a cave in County Clare. It is not until about 8000 BC, that more sustained occupation of the island has been shown, with evidence for Mesolithic communities around the island; these Mesolithic communities lived as hunter-gatherers across the island until about 4000 BC. Some time before 4000 BC, Neolithic settlers arrived introducing cereal cultivars, domesticated animals such as cattle and sheep, large timber building, stone monuments; the earliest evidence for farming in Ireland or Great Britain is from Co.. Kerry, where a flint knife, cattle bones and a sheep's tooth were carbon-dated to c. 4350 BC. Field systems were developed in different parts of Ireland, including at the Céide Fields, preserved beneath a blanket of peat in present-day Tyrawley.
An extensive field system, arguably the oldest in the world, consisted of small divisions separated by dry-stone walls. The fields were farmed for several centuries between 3500 BC and 3000 BC. Wheat and barley were the principal crops; the Bronze Age – defined by the use of metal – began around 2500 BC, with technology changing people's everyday lives during this period through innovations such as the wheel. According to John T. Koch and others, Ireland in the Late Bronze Age was part of a maritime trading-network culture called the Atlantic Bronze Age that included Britain, western France and Iberia, that this is where Celtic languages developed; this contrasts with the traditional view that their origin lies in mainland Europe with the Hallstatt culture. During the Iron Age, a Celtic language and culture emerged in Ireland. How and when the island became Celtic has been debated for close to a century, with the migrations of the Celts being one of the more enduring themes of archaeological and linguistic studies.
The most recent genetic research s
Muhammad was the founder of Islam. According to Islamic doctrine, he was a prophet, sent to present and confirm the monotheistic teachings preached by Adam, Moses and other prophets, he is viewed as the final prophet of God in all the main branches of Islam, though some modern denominations diverge from this belief. Muhammad united Arabia into a single Muslim polity, with the Quran as well as his teachings and practices forming the basis of Islamic religious belief. Born 570 CE in the Arabian city of Mecca, Muhammad was orphaned at the age of six, he was raised under the care of his paternal grandfather Abd al-Muttalib, upon his death, by his uncle Abu Talib. In years he would periodically seclude himself in a mountain cave named Hira for several nights of prayer; when he was 40, Muhammad reported being visited by Gabriel in the cave, receiving his first revelation from God. Three years in 610, Muhammad started preaching these revelations publicly, proclaiming that "God is One", that complete "submission" to God is the right way of life, that he was a prophet and messenger of God, similar to the other prophets in Islam.
The followers of Muhammad were few in number, experienced hostility from Meccan polytheists. He sent some of his followers to Abyssinia in 615 to shield them from prosecution, before he and his followers migrated from Mecca to Medina in 622; this event, the Hijra, marks the beginning of the Islamic calendar known as the Hijri Calendar. In Medina, Muhammad united the tribes under the Constitution of Medina. In December 629, after eight years of intermittent fighting with Meccan tribes, Muhammad gathered an army of 10,000 Muslim converts and marched on the city of Mecca; the conquest went uncontested and Muhammad seized the city with little bloodshed. In 632, a few months after returning from the Farewell Pilgrimage, he died. By the time of his death, most of the Arabian Peninsula had converted to Islam; the revelations, which Muhammad reported receiving until his death, form the verses of the Quran, regarded by Muslims as the verbatim "Word of God" and around which the religion is based. Besides the Quran, Muhammad's teachings and practices, found in the Hadith and sira literature, are upheld and used as sources of Islamic law.
The name Muhammad appears four times in the Quran. The Quran addresses Muhammad in the second person by various appellations. Muhammad is sometimes addressed by designations deriving from his state at the time of the address: thus he is referred to as the enwrapped in Quran 73:1 and the shrouded in Quran 74:1. In Sura Al-Ahzab 33:40 God singles out Muhammad as the "Seal of the prophets", or the last of the prophets; the Quran refers to Muhammad as Aḥmad "more praiseworthy". The name Abū al-Qāsim Muḥammad ibn ʿAbd Allāh ibn ʿAbd al-Muṭṭalib ibn Hāshim, begins with the kunya Abū, which corresponds to the English, father of; the Quran is the central religious text of Islam. Muslims believe; the Quran, provides minimal assistance for Muhammad's chronological biography. Important sources regarding Muhammad's life may be found in the historic works by writers of the 2nd and 3rd centuries of the Muslim era; these include traditional Muslim biographies of Muhammad, which provide additional information about Muhammad's life.
The earliest surviving written sira is Ibn Ishaq's Life of God's Messenger written c. 767 CE. Although the work was lost, this sira was used at great length by Ibn Hisham and to a lesser extent by Al-Tabari. However, Ibn Hisham admits in the preface to his biography of Muhammad that he omitted matters from Ibn Ishaq's biography that "would distress certain people". Another early history source is the history of Muhammad's campaigns by al-Waqidi, the work of his secretary Ibn Sa'd al-Baghdadi. Many scholars accept these early biographies as authentic. Recent studies have led scholars to distinguish between traditions touching legal matters and purely historical events. In the legal group, traditions could have been subject to invention while historic events, aside from exceptional cases, may have been only subject to "tendential shaping". Other important sources include the hadith collections, accounts of the verbal and physical teachings and traditions of Muhammad. Hadiths were compiled several generations after his death by followers including Muhammad al-Bukhari, Muslim ibn al-Hajjaj, Muhammad ibn Isa at-Tirmidhi, Abd ar-Rahman al-Nasai, Abu Dawood, Ibn Majah, Malik ibn Anas, al-Daraqutni.
Some Western academics cautiously view the hadith collections as accurate historical sources. Scholars such as Madelung do not reject the narrations which have been compiled in periods, but judge them in the context of history and on the basis of their compatibility with the events and figures. Muslim scholars on the other hand place a greater emph
The Roman Empire was the post-Roman Republic period of the ancient Roman civilization. Ruled by emperors, it had large territorial holdings around the Mediterranean Sea in Europe, North Africa, the Middle East, the Caucasus. From the constitutional reforms of Augustus to the military anarchy of the third century, the Empire was a principate ruled from the city of Rome; the Roman Empire was ruled by multiple emperors and divided in a Western Roman Empire, based in Milan and Ravenna, an Eastern Roman Empire, based in Nicomedia and Constantinople. Rome remained the nominal capital of both parts until 476 AD, when Odoacer deposed Romulus Augustus after capturing Ravenna and the Roman Senate sent the imperial regalia to Constantinople; the fall of the Western Roman Empire to barbarian kings, along with the hellenization of the Eastern Roman Empire into the Byzantine Empire, is conventionally used to mark the end of Ancient Rome and the beginning of the Middle Ages. The previous Republic, which had replaced Rome's monarchy in the 6th century BC, became destabilized in a series of civil wars and political conflict.
In the mid-1st century BC Julius Caesar was appointed as perpetual dictator and assassinated in 44 BC. Civil wars and proscriptions continued, culminating in the victory of Octavian, Caesar's adopted son, over Mark Antony and Cleopatra at the Battle of Actium in 31 BC; the following year Octavian conquered Ptolemaic Egypt, ending the Hellenistic period that had begun with the conquests of Alexander the Great of Macedon in the 4th century BC. Octavian's power was unassailable and in 27 BC the Roman Senate formally granted him overarching power and the new title Augustus making him the first emperor; the first two centuries of the Empire were a period of unprecedented stability and prosperity known as the Pax Romana. It reached its greatest territorial expanse during the reign of Trajan. A period of increasing trouble and decline began with the reign of Commodus. In the 3rd century, the Empire underwent a crisis that threatened its existence, but was reunified under Aurelian. In an effort to stabilize the Empire, Diocletian set up two different imperial courts in the Greek East and Latin West.
Christians rose to power in the 4th century following the Edict of Milan in 313 and the Edict of Thessalonica in 380. Shortly after, the Migration Period involving large invasions by Germanic peoples and the Huns of Attila led to the decline of the Western Roman Empire. With the fall of Ravenna to the Germanic Herulians and the deposition of Romulus Augustulus in 476 AD by Odoacer, the Western Roman Empire collapsed and it was formally abolished by emperor Zeno in 480 AD; the Eastern Roman Empire, known in the post-Roman West as the Byzantine Empire, collapsed when Constantinople fell to the Ottoman Turks of Mehmed II in 1453. Due to the Roman Empire's vast extent and long endurance, the institutions and culture of Rome had a profound and lasting influence on the development of language, architecture, philosophy and forms of government in the territory it governed Europe; the Latin language of the Romans evolved into the Romance languages of the medieval and modern world, while Medieval Greek became the language of the Eastern Roman Empire.
Its adoption of Christianity led to the formation of Christendom during the Middle Ages. Greek and Roman art had a profound impact on the late medieval Italian Renaissance, while Rome's republican institutions influenced the political development of republics such as the United States and France; the corpus of Roman law has its descendants in many legal systems of the world today, such as the Napoleonic Code. Rome's architectural tradition served as the basis for Neoclassical architecture. Rome had begun expanding shortly after the founding of the republic in the 6th century BC, though it did not expand outside the Italian peninsula until the 3rd century BC, it was an "empire" long before it had an emperor. The Roman Republic was not a nation-state in the modern sense, but a network of towns left to rule themselves and provinces administered by military commanders, it was ruled, not by annually elected magistrates in conjunction with the senate. For various reasons, the 1st century BC was a time of political and military upheaval, which led to rule by emperors.
The consuls' military power rested in the Roman legal concept of imperium, which means "command". Successful consuls were given the honorary title imperator, this is the origin of the word emperor since this title was always bestowed to the early emperors upon their accession. Rome suffered a long series of internal conflicts and civil wars from the late second century BC onward, while extending its power beyond Italy; this was the period of the Crisis of the Roman Republic. Towards the end of this era, in 44 BC, Julius Caesar was perpetual dictator before being assassinated; the faction of his assassins was driven from Rome and defeated at the Battle of Philippi in 42 BC by an army led by Mark Antony and Caesar's adopted son Octavian. Antony and Octavian's division of the Roman world between themselves did not last and Octavian's forces defeated those of Mark Antony and Cleopatra at the Battle of Actium in 31 BC, ending the Final War of the Roman Republic. In 27 BC the Senate and People of Rome made Octavian princeps ("first citi
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
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
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