Province of Guadalajara
Guadalajara is a province of central/north-central Spain, in the northern part of the autonomous community of Castilla–La Mancha. As of 2013 it had a population of 257,723 people; the population of the province has grown in the last 10 years. The province has been inhabited since the Paleolithic as evidenced by stone tools found on the banks of the Henares and Linares rivers. There are numerous prehistoric cave paintings in the Cueva de los Casares in Riba de Saelices while Megalithic tombs from the 4th millennium B. C. have been found at various sites in the province including Alcolea del Pinar. There are remains of several Bronze Age settlements along the river banks in the area, notably that in Loma del Lomo in Cogolludo as well as a late bronze age settlement in Mojares; the Celtiberians civilized the territory during the late Iron Age between the 6th and 3rd centuries B. C. various tribes establishing themselves in Sigüenza and Termancia in the north and further south around Molina. In addition to raising livestock and breeding horses, they created many fortified towns and villages as well as castles.
Between 143 and 133 B. C. the Romans initiated their battles to conquer Spain which continued until 94 B. C, they brought agriculture and commerce to the region, facilitating communications with roads and bridges. The most important Roman city was Segontia although they built a town wall around Luzaga where there were large public buildings; the Visigoths, with their capital at Toledo, were dominant in the area around the 6th and 7th centuries A. D. bringing Christianity and Germanic law into the region. In 578, King Leovigild founded Recópolis on the River Tagus with a palace; the Moors arrived in the area in c. 711, establishing Islamic rule for some four centuries until the early 13th century. Their most important contribution was founding of the capital, established by the Berber captain al-Faray, remembered for overcoming the Christians in the 9th century; the territory now covered by the Province of Guadalajara was part of the Moors' Marca Media. Sparsely populated, the most important towns were Atienza, Jadraque and Sigüenza.
Following the dismemberment of the Caliphate of Córdoba, Toledo gained independence in 1018, reaching its zenith under Yahya-al-Mamun who reigned from 1043 to 1075. Following his death, pressure from King Alfonso VI of León and Castile led to the beginning of Christian conquest of the region in 1085. By the early 12th century, Molina, La Serrania, Sigüenza and the Tagus Valley were retrieved leading to the establishment of the Bishopric of Sigüenza. Under Alfonso VII and Alfonso VIII, the region was repopulated with people from other parts of Castile. With the conquest of Cuenca and Alarcón at the end of the 12th century and the victory at Las Navas de Tolosa in 1212, the entire territory of Guadalajara was again in the hands of the Castilian Christians; the modern age began with the Catholic Monarchs, Isabella I of Castile and Ferdinand II of Aragon whose marriage in Valladolid in 1469 united the crowns of Castile and Aragón. They centralized the authority which had developed in the church, the military and the nobility ostensibly to earn income for fighting the infidels by reselling the territories they had gained.
In the 16th century, this practice was reinforced by Charles I and Philip II. In Guadalajara, this was the case with areas that had belonged to the military orders of Calatrava and Pastrana; the Mendozas who succeeded in acquiring substantial territories built a fortified palace in Pastrana and extended their influence over Sayatón, Escopete and Albalate. Under the Mendozas, the city of Guadalajara prospered in the 15th and 16th centuries, attracting writers and philosophers, bringing it the name la Atenas alcarreña. Encouraged by the Renaissance, Íñigo López de Mendoza, 1st Marquis of Santillana, not only built palaces and monasteries but developed a large library of Greek and Latin volumes. In the 16th century, his namesake Íñigo López de Mendoza, 4th Duke of the Infantado, went on to found an academy in the city, attracting additional writers. Pastrana prospered during the Renaissance under the leadership of Ruy Gómez de Silva with the establishment of Latin and choir schools. By the end of the 16th century, the town was famous for its Carmelite convents.
With the death of Ruy's widow, Ana de Mendoza in 1592, the nobility moved to Madrid, causing the province to lose the high status it had achieved. While the Spanish Golden Age developed in central Spain during the 17th century, Guadalajara experienced an extended period of decline as the Habsburgs brought about increased centralization. In the early 18th century, under the War of the Spanish Succession, the city of Guadalajara and the province's main towns all suffered considerable damage. In 1719, a royal textile factory was established in Guadalajara, bringing workers not only from across Spain but from the rest of Europe the Netherlands; the factory prospered throughout the 18th century but was closed in the early 19th century as a result of the War of Spanish Independence. During the War of Independence, French troops caused extensive damage to towns in the province Molina where over 600 buildings were destroyed by fire; when the city of Guadalajara was liberated in 1813, it was left in a devastated and poverty-stricken state.
Conditions improved in 1840 with the establishment of the Academy of Military Engineering in the former textile factory. Further military installations followed, culminating at the end of the century in the establishment of the Airshi
Aragon
Aragon is an autonomous community in Spain, coextensive with the medieval Kingdom of Aragon. Located in northeastern Spain, the Aragonese autonomous community comprises three provinces: Huesca and Teruel, its capital is Zaragoza. The current Statute of Autonomy declares Aragon a historic nationality of Spain. Covering an area of 47720 km2, the region's terrain ranges diversely from permanent glaciers to verdant valleys, rich pasture lands and orchards, through to the arid steppe plains of the central lowlands. Aragon is home to many rivers—most notably, the river Ebro, Spain's largest river in volume, which runs west-east across the entire region through the province of Zaragoza, it is home to the highest mountains of the Pyrenees. As of January 2016, the population of Aragon was 1308563, with over half of it living in its capital city, Zaragoza. During the same year, the economy of Aragon generates a GDP of €34687 million, which represents 3.1% of Spain's national GDP, is 6th in per capita production behind Madrid, Basque Country, Catalonia and La Rioja.
In addition to its three provinces, Aragon is subdivided into counties. All comarcas of Aragon have a rich geopolitical and cultural history from its pre-Roman and Roman days, four centuries of Islamic period as Marca Superior of Al-Andalus or kingdom of Saraqusta, as lands that once belonged to the Frankish Marca Hispanica, counties that formed the Kingdom of Aragon and the Crown of Aragon; the current coat of arms of Aragon is composed of the four barracks and is attested for the first time in 1499, consolidating since the Early Modern Ages to take root decisively in the 19th century and be approved, according to precept, by the Real Academia de la Historia in 1921. The first quartering appears at the end of the 15th century and commemorates, according to traditional interpretation, the legendary kingdom of Sobrarbe; this emblem of gules and gold was used in seals, banners and standards indistinctly, not being but a familiar emblem that denoted the authority as King of Aragon until, with the birth of Modern State, began to be a territorial symbol.
The current flag was approved in 1984, with the provisions of Article 3 of the Statute of Autonomy of Aragon, the flag is the traditional of the four horizontal red bars on a yellow background with the coat of arms of Aragon shifted towards the flagpole. The bars of Aragon, common historic element of the current four autonomous communities that once were integrated into the Crown of Aragon, present in the third quartering of the coat of arms of Spain; the anthem of Aragon was regulated in 1989 with music by the Aragonese composer Antón García Abril that combines the old Aragonese musical tradition with popular musical elements within a modern conception. The lyrics were elaborated by the Aragonese poets Ildefonso Manuel Gil, Ángel Guinda, Rosendo Tello and Manuel Vilas and highlights within its poetic framework, values such as freedom, reason, open land... that represent the expression of Aragon as a people. The Day of Aragon is celebrated on April 23 and commemorates Saint George, patron of the Kingdom of Aragon since the 15th century.
It appears in Article 3 of the Statute of Autonomy of Aragon since 1984. Institutional acts such as the delivery of the Aragon Awards by the Government of Aragon or the composition of a flag of Aragon of flowers, with the collaboration of citizens, in the Plaza de Aragón square of Zaragoza; the area of Aragon is 47720 km2 of which 15636 km2 belong to the province of Huesca, 17275 km2 to the province of Zaragoza and 14810 km2 to the province of Teruel. The total represents a 9.43% of the surface of Spain, being thus the fourth autonomous community in size behind Castile and León, Castile-La Mancha. It is located in the northeast of the Iberian Peninsula, at a latitude between 39º and 43º'N in the temperate zone of the Earth, its boundaries and borders are in the north with France, the regions of, in the west with the autonomous communities of Castile-La Mancha, Castile and León, La Rioja and Navarre and in the east with the autonomous communities of Catalonia and Valencian Community. The orography of the community has as central axis the Ebro valley which tr
Birefringence
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent; the birefringence is quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are birefringent, as are plastics under mechanical stress. Birefringence is responsible for the phenomenon of double refraction whereby a ray of light, when incident upon a birefringent material, is split by polarization into two rays taking different paths; this effect was first described by the Danish scientist Rasmus Bartholin in 1669, who observed it in calcite, a crystal having one of the strongest birefringences. However it was not until the 19th century that Augustin-Jean Fresnel described the phenomenon in terms of polarization, understanding light as a wave with field components in transverse polarizations. A mathematical description of wave propagation in a birefringent medium is presented below.
Following is a qualitative explanation of the phenomenon. The simplest type of birefringence is described as uniaxial, meaning that there is a single direction governing the optical anisotropy whereas all directions perpendicular to it are optically equivalent, thus rotating the material around this axis does not change its optical behavior. This special direction is known as the optic axis of the material. Light propagating parallel to the optic axis is governed by a refractive index no. Light whose polarization is in the direction of the optic axis sees an optical index ne. For any ray direction there is a linear polarization direction perpendicular to the optic axis, this is called an ordinary ray. However, for ray directions not parallel to the optic axis, the polarization direction perpendicular to the ordinary ray's polarization will be in the direction of the optic axis, this is called an extraordinary ray. I.e. when unpolarized light enters an uniaxial birefringent material it is split into two beams travelling different directions.
The ordinary ray will always experience a refractive index of no, whereas the refractive index of the extraordinary ray will be in between no and ne, depending on the ray direction as described by the index ellipsoid. The magnitude of the difference is quantified by the birefringence: Δ n = n e − n o; the propagation of the ordinary ray is described by no as if there were no birefringence involved. However the extraordinary ray, as its name suggests, propagates unlike any wave in a homogenous optical material, its refraction at a surface can be understood using the effective refractive index. However it is in fact an inhomogeneous wave whose power flow is not in the direction of the wave vector; this causes an additional shift in that beam when launched at normal incidence, as is popularly observed using a crystal of calcite as photographed above. Rotating the calcite crystal will cause one of the two images, that of the extraordinary ray, to rotate around that of the ordinary ray, which remains fixed.
When the light propagates either along or orthogonal to the optic axis, such a lateral shift does not occur. In the first case, both polarizations see the same effective refractive index, so there is no extraordinary ray. In the second case the extraordinary ray propagates at a different phase velocity but is not an inhomogeneous wave. A crystal with its optic axis in this orientation, parallel to the optical surface, may be used to create a waveplate, in which there is no distortion of the image but an intentional modification of the state of polarization of the incident wave. For instance, a quarter-wave plate is used to create circular polarization from a linearly polarized source; the case of so-called biaxial crystals is more complex. These are characterized by three refractive indices corresponding to three principal axes of the crystal. For most ray directions, both polarizations would be classified as extraordinary rays but with different effective refractive indices. Being extraordinary waves, the direction of power flow is not identical to the direction of the wave vector in either case.
The two refractive indices can be determined using the index ellipsoids for given directions of the polarization. Note that for biaxial crystals the index ellipsoid will not be an ellipsoid of revolution but is described by three unequal principle refractive indices nα, nβ and nγ, thus there is no axis. Although there is no axis of symmetry, there are two optical axes or binormals which are defined as directions along which light may propagate without birefringence, i.e. directions along which the wavelength is independent of polarization. For this reason, birefringent materials with three distinct refractive indices are called biaxial. Additionally, there are two distinct axes known as optical ray axes or biradials along which the group velocity of the light is independent of polarization; when an arbitrary beam of light strikes the surface of a b
Iridescence
Iridescence is the phenomenon of certain surfaces that appear to change colour as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, butterfly wings and seashells, as well as certain minerals, it is created by structural coloration. Pearlescence is a related effect where some or all of the reflected light is white, where iridescent effects produce only other colours; the term pearlescent is used to describe certain paint finishes in the automotive industry, which produce iridescent effects. The word iridescence is derived in part from the Greek word ἶρις îris, meaning rainbow, is combined with the Latin suffix -escent, meaning "having a tendency toward". Iris in turn derives from the goddess Iris of Greek mythology, the personification of the rainbow and acted as a messenger of the gods. Goniochromism is derived from the Greek words gonia, meaning "angle", chroma, meaning "colour". Iridescence is an optical phenomenon of surfaces in which hue changes with the angle of observation and the angle of illumination.
It is caused by multiple reflections from two or more semi-transparent surfaces in which phase shift and interference of the reflections modulates the incidental light. The thickness of the layers of the material determines the interference pattern. Iridescence can for example be due to thin-film interference, the functional analogue of selective wavelength attenuation as seen with the Fabry–Pérot interferometer, can be seen in oil films on water and soap bubbles. Iridescence is found in plants and many other items; the range of colours of natural iridescent objects can be narrow, for example shifting between two or three colours as the viewing angle changes, or a wide range of colours can be observed. Iridescence can be created by diffraction; this is found in items like DVDs, some types of prisms, or cloud iridescence. In the case of diffraction, the entire rainbow of colours will be observed as the viewing angle changes. In biology, this type of iridescence results from the formation of diffraction gratings on the surface, such as the long rows of cells in striated muscle, or the specialized abdominal scales of peacock spider Maratus robinsoni and M. chrysomelas.
Some types of flower petals can generate a diffraction grating, but the iridescence is not visible to humans and flower-visiting insects as the diffraction signal is masked by the coloration due to plant pigments. In biological uses, colours produced other than with pigments or dyes are called structural coloration. Microstructures multilayered, are used to produce bright but sometimes non-iridescent colours: quite elaborate arrangements are needed to avoid reflecting different colours in different directions. Structural coloration has been understood in general terms since Robert Hooke's 1665 book Micrographia, where Hooke noted that since the iridescence of a peacock's feather was lost when it was plunged into water, but reappeared when it was returned to the air, pigments could not be responsible, it was found that iridescence in the peacock is due to a complex photonic crystal. Pearlescence is an effect has a similar cause. Structures within a surface cause light to be reflected back, but in the case of pearlescence some or all of the light is white.
Artificial pigments and paints showing an iridescent effect are described as pearlescent, for example when used for car paints. Arthropods and molluscs: Chordates: The feathers of birds such as kingfishers, birds-of-paradise, parrots, grackles and peacocks are iridescent; the lateral line on the neon tetra is iridescent. A single iridescent species of gecko, Cnemaspis kolhapurensis, was identified in India in 2009; the tapetum lucidum, present in the eyes of many vertebrates, is iridescent. Plants: Many groups of plants have developed iridescence as an adaptation to use more light in dark environments such as the lower levels of tropical forests; the leaves of Southeast Asia's Begonia pavonina, or peacock begonia, appear iridescent azure to human observers due to each leaf's thinly layered photosynthetic structures called iridoplasts that absorb and bend light much like a film of oil over water. Iridescences based on multiple layers of cells are found in the lycophyte Selaginella and several species of ferns.
Meat: Minerals and compounds: Man-made objects: Nanocellulose is sometimes iridescent, as are thin films of gasoline and some other hydrocarbons and alcohols when floating on water. To create jewelry with crystal glass that lets light refract in a rainbow spectrum, Swarovski coats some of its products with special metallic chemical coatings and for example his Aurora Borealis gives the surface a rainbow appearance. A 2.2 MB GIF animation of a morpho butterfly showing iridescence "Article on butterfly iridescence"
Slovakia
Slovakia the Slovak Republic, is a landlocked country in Central Europe. It is bordered by Poland to the north, Ukraine to the east, Hungary to the south, Austria to the west, the Czech Republic to the northwest. Slovakia's territory spans about 49,000 square kilometres and is mountainous; the population is over 5.4 million and consists of Slovaks. The capital and largest city is Bratislava, the second largest city is Košice; the official language is Slovak. The Slavs arrived in the territory of present-day Slovakia in the 6th centuries. In the 7th century, they played a significant role in the creation of Samo's Empire and in the 9th century established the Principality of Nitra, conquered by the Principality of Moravia to establish Great Moravia. In the 10th century, after the dissolution of Great Moravia, the territory was integrated into the Principality of Hungary, which would become the Kingdom of Hungary in 1000. In 1241 and 1242, much of the territory was destroyed by the Mongols during their invasion of Central and Eastern Europe.
The area was recovered thanks to Béla IV of Hungary who settled Germans which became an important ethnic group in the area in what are today parts of central and eastern Slovakia. After World War I and the dissolution of the Austro-Hungarian Empire, the Czechoslovak National Council established Czechoslovakia. A separate Slovak Republic existed during World War II as a totalitarian, clero-fascist one-party client state of Nazi Germany. At the end of World War II, Czechoslovakia was re-established as an independent country. A coup in 1948 ushered in a totalitarian one-party state under the Communist regime during whose rule the country existed as a satellite of the Soviet Union. Attempts for liberalization of communism in Czechoslovakia culminated in the Prague Spring, crushed by the Warsaw Pact invasion of Czechoslovakia in August 1968. In 1989, the Velvet Revolution ended the Communist rule in Czechoslovakia peacefully. Slovakia became an independent state on 1 January 1993 after the peaceful dissolution of Czechoslovakia, sometimes known as the Velvet Divorce.
Slovakia is a developed country, with a high-income advanced economy and a high Human Development Index, a high standard of living and performs favourably in measurements of civil liberties, press freedom, internet freedom, democratic governance and peacefulness. The country maintains a combination of market economy with a comprehensive social security system. Citizens of Slovakia are provided with universal health care, free education and one of the longest paid parental leave in the OECD; the country joined the European Union on 1 May 2004 and joined the Eurozone on 1 January 2009. Slovakia is a member of the Schengen Area, NATO, the United Nations, the OECD, the WTO, CERN, the OSCE, the Council of Europe and the Visegrád Group. Although regional income inequality is high, 90% of citizens own their homes. In 2018, Slovak citizens had visa-free or visa-on-arrival access to 179 countries and territories, ranking the Slovak passport 10th in the world; as part of Eurozone, Slovak legal tender is the world's 2nd-most-traded currency.
Slovakia is the world's largest per-capita car producer with a total of 1,040,000 cars manufactured in the country in 2016 alone and the 7th largest car producer in the European Union. The car industry represents 43% of Slovakia's industrial output, a quarter of its exports; the first written mention of name Slovakia is in 1586. It derives from the Czech word Slováky; the native name Slovensko derives from an older name of Slovaks Sloven what may indicate its origin before the 15th century. The original meaning was geographic, since Slovakia was a part of the multiethnic Kingdom of Hungary and did not form a separate administrative unit in this period. Radiocarbon dating puts the oldest surviving archaeological artefacts from Slovakia – found near Nové Mesto nad Váhom – at 270,000 BCE, in the Early Paleolithic era; these ancient tools, made by the Clactonian technique, bear witness to the ancient habitation of Slovakia. Other stone tools from the Middle Paleolithic era come from the Prévôt cave in Bojnice and from other nearby sites.
The most important discovery from that era is a Neanderthal cranium, discovered near Gánovce, a village in northern Slovakia. Archaeologists have found prehistoric human skeletons in the region, as well as numerous objects and vestiges of the Gravettian culture, principally in the river valleys of Nitra, Ipeľ, Váh and as far as the city of Žilina, near the foot of the Vihorlat and Tribeč mountains, as well as in the Myjava Mountains; the most well-known finds include the oldest female statue made of mammoth-bone, the famous Venus of Moravany. The statue was found in the 1940s in Moravany nad Váhom near Piešťany. Numerous necklaces made of shells from Cypraca thermophile gastropods of the Tertiary period have come from the sites of Zákovská, Podkovice and Radošina; these findings provide the most ancient evidence of commercial exchanges carried out between the Mediterranean and central Europe. The Bronze Age in the geographical territory of modern-day Slovakia went through three stages of development, stretching from 2000 to 800 BCE.
Major cultural and political development can be attributed to the significant growth in production of copper in central Slovakia and northwe
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate. The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite". Other polymorphs of calcium carbonate are the minerals vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, vaterite is less stable. Calcite is derived from the German Calcit, a term coined in the 19th century from the Latin word for lime, calx with the suffix -ite used to name minerals, it is thus etymologically related to chalk. When applied by archaeologists and stone trade professionals, the term alabaster is used not just as in geology and mineralogy, where it is reserved for a variety of gypsum. In publications, two different sets of Miller indices are used to describe directions in calcite crystals - the hexagonal system with three indices h, k, l and the rhombohedral system with four indices h, k, l, i. To add to the complications, there are two definitions of unit cell for calcite.
One, an older "morphological" unit cell, was inferred by measuring angles between faces of crystals and looking for the smallest numbers that fit. A "structural" unit cell was determined using X-ray crystallography; the morphological unit cell has approximate dimensions a = 10 Å and c = 8.5 Å, while for the structural unit cell they are a = 5 Å and c = 17 Å. For the same orientation, c must be multiplied by 4 to convert from morphological to structural units; as an example, the cleavage is given as "perfect on " in morphological coordinates and "perfect on " in structural units. Twinning and crystal forms are always given in morphological units. Over 800 forms of calcite crystals have been identified. Most common are scalenohedra, with faces in the hexagonal directions or directions. Habits include acute to tabular forms, prisms, or various scalenohedra. Calcite exhibits several twinning types adding to the variety of observed forms, it may occur as fibrous, lamellar, or compact. A fibrous, efflorescent form is known as lublinite.
Cleavage is in three directions parallel to the rhombohedron form. Its fracture is difficult to obtain. Scalenohedral faces are chiral and come in pairs with mirror-image symmetry. Rhombohedral faces are achiral, it has a defining Mohs hardness of 3, a specific gravity of 2.71, its luster is vitreous in crystallized varieties. Color is white or none, though shades of gray, orange, green, violet, brown, or black can occur when the mineral is charged with impurities. Calcite is transparent to opaque and may show phosphorescence or fluorescence. A transparent variety called. Acute scalenohedral crystals are sometimes referred to as "dogtooth spar" while the rhombohedral form is sometimes referred to as "nailhead spar". Single calcite crystals display; this strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. The birefringent effect was first described by the Danish scientist Rasmus Bartholin in 1669. At a wavelength of ≈590 nm calcite has ordinary and extraordinary refractive indices of 1.658 and 1.486, respectively.
Between 190 and 1700 nm, the ordinary refractive index varies between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4. Calcite, like most carbonates, will dissolve with most forms of acid. Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, dissolved ion concentrations. Although calcite is insoluble in cold water, acidity can cause dissolution of calcite and release of carbon dioxide gas. Ambient carbon dioxide, due to its acidity, has a slight solubilizing effect on calcite. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures. When conditions are right for dissolution, the removal of calcite can increase the porosity and permeability of the rock, if it continues for a long period of time may result in the formation of caves.
On a landscape scale, continued dissolution of calcium carbonate-rich rocks can lead to the expansion and eventual collapse of cave systems, resulting in various forms of karst topography. Ancient Egyptians carved many items out of calcite, relating it to their goddess Bast, whose name contributed to the term alabaster because of the close association. Many other cultures have used the material for similar carved applications. High-grade optical calcite was used in World War II for gun sights in bomb sights and anti-aircraft weaponry. Experiments have been conducted to use calcite for a cloak of invisibility. Microbiologically precipitated calcite has a wide range of applications, such as soil remediation, soil stabilization and concrete repair. Calcite, obtained from an 80 kg sample of Carrara marble, is used as the IAEA-603 isotopic standard in mass spectrometry for the calibration of δ18O and δ13C. Calcite is a common constituent
Lustre (mineralogy)
Lustre or luster is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin lux, meaning "light", implies radiance, gloss, or brilliance. A range of terms are used to describe lustre, such as earthy, metallic and silky; the term vitreous refers to a glassy lustre. A list of these terms is given below. Lustre varies over a wide continuum, so there are no rigid boundaries between the different types of lustre; the terms are combined to describe intermediate types of lustre. Some minerals exhibit unusual optical phenomena, such as asterism or chatoyancy. A list of such phenomena is given below. Adamantine minerals possess a superlative lustre, most notably seen in diamond; such minerals are transparent or translucent, have a high refractive index. Minerals with a true adamantine lustre are uncommon, with examples being cerussite and cubic zirconia. Minerals with a lesser degree of lustre are referred to as subadamantine, with some examples being garnet and corundum.
Dull minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, approximating a Lambertian reflector. An example is kaolinite. A distinction is sometimes drawn between dull minerals and earthy minerals, with the latter being coarser, having less lustre. Greasy minerals resemble grease. A greasy lustre occurs in minerals containing a great abundance of microscopic inclusions, with examples including opal and cordierite, jadeite. Many minerals with a greasy lustre feel greasy to the touch. Metallic minerals have the lustre of polished metal, with ideal surfaces will work as a reflective surface. Examples include galena and magnetite. Pearly minerals consist of thin transparent co-planar sheets. Light reflecting from these layers give them a lustre reminiscent of pearls; such minerals possess perfect cleavage, with examples including stilbite. Resinous minerals have the appearance of chewing gum or plastic. A principal example is amber, a form of fossilized resin.
Silky minerals have a parallel arrangement of fine fibres, giving them a lustre reminiscent of silk. Examples include asbestos and the satin spar variety of gypsum. A fibrous lustre has a coarser texture. Submetallic minerals are duller and less reflective. A submetallic lustre occurs in near-opaque minerals with high refractive indices, such as sphalerite and cuprite. Vitreous minerals have the lustre of glass; this type of lustre is one of the most seen, occurs in transparent or translucent minerals with low refractive indices. Common examples include calcite, topaz, beryl and fluorite, among others. Waxy minerals have a lustre resembling wax. Examples include chalcedony. Asterism is the display of a star-shaped luminous area, it is seen in some rubies, where it is caused by impurities of rutile. It can occur in garnet and spinel. Aventurescence is a reflectance effect like that of glitter, it arises from minute, preferentially oriented mineral platelets within the material. These platelets are so numerous that they influence the material's body colour.
In aventurine quartz, chrome-bearing fuchsite makes for a green stone and various iron oxides make for a red stone. Chatoyant minerals display luminous bands; such minerals are composed of parallel fibers, which reflect light into a direction perpendicular to their orientation, thus forming narrow bands of light. The most famous examples are tiger's eye and cymophane, but the effect may occur in other minerals such as aquamarine and tourmaline. Color change is most found in alexandrite, a variety of chrysoberyl gemstones. Other gems occur in color-change varieties, including sapphire, spinel. Alexandrite displays a color change dependent upon light, along with strong pleochroism; the gem results from small-scale replacement of aluminium by chromium oxide, responsible for alexandrite's characteristic green to red color change. Alexandrite from the Ural Mountains in Russia is green by red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light.
The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is rare. Iridescence is the'play' or'fire' of rainbow-coloured light caused by thin regular structures or layers beneath the surface of a gemstone. Similar to a thin film of oil on water, these layers interfere with the rays of reflected light, reinforcing some colours and cancelling others. Iridescence is seen at its best in precious opal. Schiller, from German for "color play", is the metallic iridescence originating from below the surface of a stone that occurs when light is reflected between layers of minerals, it is seen in moonstone and labradorite and is similar to adularescence and aventurescence
