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58-448: This brooch is a large composite, slightly convex disc, made of two plates of gold connected by a band of beaded gold wire filigree. The setting design is a modified cross pattern with a central boss (raised ornament) and four additional, smaller bosses, decorated with white shell. Constructed in gold, and inlaid with blue glass, white shell, pearl and garnet , the brooch is 8 centimetres (3.1 in) in diameter. The brooch has been dated to

116-439: A Curie temperature of 550  K . Yttrium iron garnet can be made into YIG spheres , which serve as magnetically tunable filters and resonators for microwave frequencies. Lutetium aluminium garnet (LuAG), Al 5 Lu 3 O 12 , is an inorganic compound with a unique crystal structure primarily known for its use in high-efficiency laser devices. LuAG is also useful in the synthesis of transparent ceramics . LuAG

174-409: A Faraday rotator material with excellent transparency properties and is very resistant to laser damage. TGG can be used in optical isolators for laser systems, in optical circulators for fiber optic systems, in optical modulators , and in current and magnetic field sensors. Another example is gadolinium gallium garnet (GGG) , Gd 3 Ga 2 (GaO 4 ) 3 which is synthesized for use as

232-415: A Lambertian reflector . An example is kaolinite . A distinction is sometimes drawn between dull minerals and earthy minerals, with the latter being coarser, and having even less lustre. Greasy minerals resemble fat or grease. A greasy lustre often occurs in minerals containing a great abundance of microscopic inclusions, with examples including opal and cordierite , jadeite . Many minerals with

290-435: A "vitreous greasy" lustre). Some minerals exhibit unusual optical phenomena, such as asterism (the display of a star-shaped luminous area) or chatoyancy (the display of luminous bands, which appear to move as the specimen is rotated). A list of such phenomena is given below. Adamantine minerals possess a superlative lustre, which is most notably seen in diamond . Such minerals are transparent or translucent, and have

348-486: A Gore Mountain garnet to obtain a Lu-Hf isochron age of 1046.6 ± 6 Ma. It is therefore concluded with confidence that the garnets formed at 1049 ± 5 Ma, the average of the three determinations. This is also the local age of peak metamorphism in the 1090–1040 Ma Ottawan phase of the Grenvillian orogeny and serves as a critical data point in ascertaining the evolution of the megacrystic garnet deposits. Red garnets were

406-670: A U-Pb geochronometer , to date the age of crystallization as well as a thermochronometer in the (U-Th)/He system to date timing of cooling below a closure temperature . Garnets can be chemically altered and most often alter to serpentine, talc , and chlorite . The open-pit Barton Garnet Mine, located at Gore Mountain in the Adirondack Mountains , yields the world's largest single crystals of garnet; diameters range from 5 to 35 cm and commonly average 10–18 cm. Gore Mountain garnets are unique in many respects, and considerable effort has been made to determine

464-458: A glassy lustre. A list of these terms is given below. Lustre varies over a wide continuum, and so there are no rigid boundaries between the different types of lustre. (For this reason, different sources can often describe the same mineral differently. This ambiguity is further complicated by lustre's ability to vary widely within a particular mineral species). The terms are frequently combined to describe intermediate types of lustre (for example,

522-420: A gold pendant, a glass palm cup, a pair of silver safety-pin brooches, and a bronze hanging-bowl. The ancient burial mound was middle-sized, while the grave was much larger than normal: 10 feet (3.0 m) long by 8 feet (2.4 m) wide by 6 feet (1.8 m) deep. Constructed in gold, with garnet, shell and blue glass settings, the brooch was in excellent condition when it was discovered. Faussett's notes at

580-510: A greasy lustre also feel greasy to the touch. Metallic (or splendent ) minerals have the lustre of polished metal, and with ideal surfaces will work as a reflective surface . Examples include galena , pyrite 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 muscovite and stilbite . Resinous minerals have

638-466: A high refractive index (of 1.9 or more). Minerals with a true adamantine lustre are uncommon, with examples including cerussite , zircon , and cubic zirconia . Minerals with a lesser (but still relatively high) degree of lustre are referred to as subadamantine , with some examples being garnet and corundum . Dull (or earthy ) minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, approximating

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696-447: A pick-up response to a strong neodymium magnet separates garnet from all other natural transparent gemstones commonly used in the jewelry trade. Magnetic susceptibility measurements in conjunction with refractive index can be used to distinguish garnet species and varieties, and determine the composition of garnets in terms of percentages of end-member species within an individual gem. Almandine, sometimes incorrectly called almandite,

754-631: A reference to mela granatum or even pomum granatum (' pomegranate ', Punica granatum ), a plant whose fruits contain abundant and vivid red seed covers ( arils ), which are similar in shape, size, and color to some garnet crystals. Hessonite garnet is also named 'gomed' in Indian literature and is one of the 9 jewels in Vedic astrology that compose the Navaratna . Garnet species are found in every colour, with reddish shades most common. Blue garnets are

812-426: A result of the relatively high amounts of vanadium (about 1 wt.% V 2 O 3 ). Other varieties of color-changing garnets exist. In daylight, their color ranges from shades of green, beige, brown, gray, and blue, but in incandescent light, they appear a reddish or purplish/pink color. This is the rarest type of garnet. Because of its color-changing quality, this kind of garnet resembles alexandrite . Andradite

870-497: A substrate for liquid-phase epitaxy of magnetic garnet films for bubble memory and magneto-optical applications. The mineral garnet is commonly found in metamorphic and to a lesser extent, igneous rocks. Most natural garnets are compositionally zoned and contain inclusions. Its crystal lattice structure is stable at high pressures and temperatures and is thus found in green-schist facies metamorphic rocks including gneiss , hornblende schist , and mica schist. The composition that

928-493: Is a calcium chromium garnet with the formula Ca 3 Cr 2 (SiO 4 ) 3 . This is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite , serpentinite , and kimberlites. It is found in crystalline marbles and schists in the Ural Mountains of Russia and Outokumpu, Finland . Uvarovite is named for Count Uvaro , a Russian imperial statesman. Knorringite

986-470: Is a calcium-iron garnet, Ca 3 Fe 2 (SiO 4 ) 3 , is of variable composition and may be red, yellow, brown, green or black. The recognized varieties are demantoid (green), melanite (black), and topazolite (yellow or green). The red-brown translucent variety of colophonite is recognized as a partially obsolete name. Andradite is found in skarns and in deep-seated igneous rocks like syenite as well as serpentines and greenschists . Demantoid

1044-421: Is a magnesium-chromium garnet species with the formula Mg 3 Cr 2 (SiO 4 ) 3 . Pure endmember knorringite never occurs in nature. Pyrope rich in the knorringite component is only formed under high pressure and is often found in kimberlites . It is used as an indicator mineral in the search for diamonds . Also known as rare-earth garnets. The crystallographic structure of garnets has been expanded from

1102-487: Is a violet-red shade and has been called rhodolite , Greek for "rose". In chemical composition it may be considered as essentially an isomorphous mixture of pyrope and almandine, in the proportion of two parts pyrope to one part almandine. Pyrope has tradenames some of which are misnomers ; Cape ruby , Arizona ruby , California ruby , Rocky Mountain ruby , and Bohemian ruby from the Czech Republic . Pyrope

1160-500: Is an indicator mineral for high-pressure rocks. Mantle -derived rocks ( peridotites and eclogites ) commonly contain a pyrope variety. Spessartine or spessartite is manganese aluminium garnet, Mn 3 Al 2 (SiO 4 ) 3 . Its name is derived from Spessart in Bavaria . It occurs most often in skarns , granite pegmatite and allied rock types, and in certain low grade metamorphic phyllites . Spessartine of an orange -yellow

1218-640: Is found in Madagascar. Violet-red spessartines are found in rhyolites in Colorado and Maine . Blue pyrope–spessartine garnets were discovered in the late 1990s in Bekily, Madagascar . This type has also been found in parts of the United States , Russia , Kenya , Tanzania , and Turkey . It changes color from blue-green to purple depending on the color temperature of viewing light, as

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1276-590: Is one of the most prized of garnet varieties. Grossular is a calcium-aluminium garnet with the formula Ca 3 Al 2 (SiO 4 ) 3 , though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from the botanical name for the gooseberry , grossularia , in reference to the green garnet of this composition that is found in Siberia . Other shades include cinnamon brown (cinnamon stone variety), red, and yellow. Because of its inferior hardness to zircon , which

1334-518: Is particularly abundant in Australia. The river sand garnet occurs as a placer deposit . Rock garnet is perhaps the garnet type used for the longest period of time. This type of garnet is produced in America, China and western India. These crystals are crushed in mills and then purified by wind blowing, magnetic separation, sieving and, if required, washing. Being freshly crushed, this garnet has

1392-399: Is particularly favored over other crystals for its high density and thermal conductivity; it has a relatively small lattice constant in comparison to the other rare-earth garnets, which results in a higher density producing a crystal field with narrower linewidths and greater energy level splitting in absorption and emission. Terbium gallium garnet (TGG) , Tb 3 Ga 5 O 12 , is

1450-531: Is particularly popular due to its consistent supplies, huge quantities and clean material. The common problems with this material are the presence of ilmenite and chloride compounds. Since the material has been naturally crushed and ground on the beaches for past centuries, the material is normally available in fine sizes only. Most of the garnet at the Tuticorin beach in south India is 80 mesh, and ranges from 56 mesh to 100 mesh size. River garnet

1508-675: Is red in color and chemically an aluminium silicate with the formula Mg 3 Al 2 (SiO 4 ) 3 , though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to black. Pyrope and spessartine gemstones have been recovered from the Sloan diamondiferous kimberlites in Colorado , from the Bishop Conglomerate and in a Tertiary age lamprophyre at Cedar Mountain in Wyoming . A variety of pyrope from Macon County , North Carolina

1566-472: Is responsible for alexandrite's characteristic green to red colour change. Alexandrite from the Ural Mountains in Russia is green by daylight and 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" colour change would be fine emerald green to fine purplish red, but this is rare. Iridescence

1624-469: Is stable at the pressure and temperature conditions of Earth's mantle is pyrope, which is often found in peridotites and kimberlites , as well as the serpentines that form from them. Garnets are unique in that they can record the pressures and temperatures of peak metamorphism and are used as geobarometers and geothermometers in the study of geothermobarometry which determines "P-T Paths", Pressure-Temperature Paths. Garnets are used as an index mineral in

1682-766: Is suitable since it is more angular in form, therefore more efficient in cutting. Garnet paper is favored by cabinetmakers for finishing bare wood. Garnet sand is also used for water filtration media. As an abrasive, garnet can be broadly divided into two categories; blasting grade and water jet grade. The garnet, as it is mined and collected, is crushed to finer grains; all pieces which are larger than 60 mesh (250 micrometers) are normally used for sand blasting. The pieces between 60 mesh (250 micrometers) and 200 mesh (74 micrometers) are normally used for water jet cutting. The remaining garnet pieces that are finer than 200 mesh (74 micrometers) are used for glass polishing and lapping. Regardless of

1740-427: Is the 'play' or 'fire' of rainbow-coloured light caused by very 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 (German, literally "shimmer"), is the metallic iridescence originating from below

1798-432: Is the display of a star-shaped luminous area. It is seen in some sapphires and rubies , where it is caused by impurities of rutile . It can also occur in garnet , diopside and spinel . Aventurescence (or aventurization ) 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 also influence

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1856-528: Is the modern gem known as carbuncle (though originally almost any red gemstone was known by this name). The term "carbuncle" is derived from the Latin meaning "live coal" or burning charcoal. The name Almandine is a corruption of Alabanda , a region in Asia Minor where these stones were cut in ancient times. Chemically, almandine is an iron-aluminium garnet with the formula Fe 3 Al 2 (SiO 4 ) 3 ;

1914-442: Is the state gemstone of New York , and grossular garnet is the state gemstone of Vermont . Garnet sand is a good abrasive , and a common replacement for silica sand in sand blasting. Alluvial garnet grains which are rounder are more suitable for such blasting treatments. Mixed with very high pressure water, garnet is used to cut steel and other materials in water jets . For water jet cutting, garnet extracted from hard rock

1972-452: 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", and generally implies radiance, gloss, or brilliance. A range of terms are used to describe lustre, such as earthy , metallic , greasy , and silky . Similarly, the term vitreous (derived from the Latin for glass , vitrum ) refers to

2030-558: Is usually occupied by divalent cations ( Ca , Mg , Fe , Mn ) and the Y site by trivalent cations ( Al , Fe , Cr ) in an octahedral / tetrahedral framework with [SiO 4 ] occupying the tetrahedra. Garnets are most often found in the dodecahedral crystal habit , but are also commonly found in the trapezohedron habit as well as the hexoctahedral habit. They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other, but are never actually cubic because, despite being isometric,

2088-522: The University of Oxford . In August 1853 Godfrey Faussett's son Bryan offered his grandfather's collection of antiquities for sale to the British Museum , only for it to be declined by the trustees. Some outcry was raised in archaeological circles without effect. In 1855, the collection, including the brooch, was sold to the antiquarian and collector, Joseph Mayer (1803-1886). Mayer donated

2146-687: The old world , including to Rome, Greece, the Middle East, Serica and Anglo Saxons; recent findings such as the Staffordshire Hoard and the pendant of the Winfarthing Woman skeleton of Norfolk confirm an established gem trade route with South India and Tamraparni (ancient Sri Lanka ), known from antiquity for its production of gemstones. Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow, and orange. In

2204-515: The 1970s until the methods of producing the more advanced simulant cubic zirconia in commercial quantities were developed. When doped with neodymium (Nd ), erbium or gadolinium YAG may be used as the lasing medium in Nd:YAG lasers , Er:YAG lasers and Gd:YAG lasers respectively. These doped YAG lasers are used in medical procedures including laser skin resurfacing , dentistry, and ophthalmology. Interesting magnetic properties arise when

2262-472: The United States it is known as the birthstone for January. It is also the birthstone of Aquarius and Capricorn in tropical astrology . The garnet family is one of the most complex in the gem world. It is not a single species, but is composed of multiple species and varieties. Almandine garnet is the state mineral of Connecticut , star garnet is the state gemstone of Idaho , garnet

2320-675: The appearance of resin , chewing gum or (smooth-surfaced) plastic. A principal example is amber , which is a form of fossilized resin. Silky minerals have a parallel arrangement of extremely fine fibres, giving them a lustre reminiscent of silk . Examples include asbestos , ulexite and the satin spar variety of gypsum . A fibrous lustre is similar, but has a coarser texture. Submetallic minerals have similar lustre to metal, but are duller and less reflective. A submetallic lustre often occurs in near-opaque minerals with very high refractive indices, such as sphalerite , cinnabar , anthracite , and cuprite . Vitreous minerals have

2378-408: The application, the larger grain sizes are used for faster work and the smaller ones are used for finer finishes. There are different kinds of abrasive garnets which can be divided based on their origin. The largest source of abrasive garnet today is garnet-rich beach sand which is quite abundant on Indian and Australian coasts and the main producers today are Australia and India. This material

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2436-403: The appropriate elements are used. In yttrium iron garnet (YIG), Y 3 Fe 2 (FeO 4 ) 3 , the five iron(III) ions occupy two octahedral and three tetrahedral sites, with the yttrium(III) ions coordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordination sites exhibit different spins , resulting in magnetic behavior. YIG is a ferrimagnetic material having

2494-765: The brooch, along with his entire collection, to the Liverpool Museum in 1867. Garnet Garnets ( / ˈ ɡ ɑːr n ɪ t / ) are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives . All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition . The different species are pyrope , almandine , spessartine , grossular (varieties of which are hessonite or cinnamon-stone and tsavorite ), uvarovite and andradite . The garnets make up two solid solution series: pyrope-almandine-spessartine (pyralspite), with

2552-403: The composition range [Mg,Fe,Mn] 3 Al 2 (SiO 4 ) 3 ; and uvarovite-grossular-andradite (ugrandite), with the composition range Ca 3 [Cr,Al,Fe] 2 (SiO 4 ) 3 . The word garnet comes from the 14th-century Middle English word gernet , meaning 'dark red'. It is borrowed from Old French grenate from Latin granatus, from granum ('grain, seed'). This is possibly

2610-467: The deep red transparent stones are often called precious garnet and are used as gemstones (being the most common of the gem garnets). Almandine occurs in metamorphic rocks like mica schists , associated with minerals such as staurolite , kyanite , andalusite , and others. Almandine has nicknames of Oriental garnet, almandine ruby, and carbuncle. Pyrope (from the Greek pyrōpós meaning "firelike")

2668-668: The delineation of isograds in metamorphic rocks. Compositional zoning and inclusions can mark the change from growth of the crystals at low temperatures to higher temperatures. Garnets that are not compositionally zoned more than likely experienced ultra high temperatures (above 700 °C) that led to diffusion of major elements within the crystal lattice, effectively homogenizing the crystal or they were never zoned. Garnets can also form metamorphic textures that can help interpret structural histories. In addition to being used to devolve conditions of metamorphism, garnets can be used to date certain geologic events. Garnet has been developed as

2726-478: The effect may also occur in other minerals such as aquamarine , moonstone and tourmaline . Colour change is most commonly found in alexandrite, a variety of chrysoberyl gemstones. Other gems also occur in colour-change varieties, including (but not limited to) sapphire , garnet , spinel . Alexandrite displays a colour change dependent upon light, along with strong pleochroism . The gem results from small-scale replacement of aluminium by chromium oxide, which

2784-441: The lustre of glass . (The term is derived from the Latin for glass, vitrum .) This type of lustre is one of the most commonly seen, and occurs in transparent or translucent minerals with relatively low refractive indices. Common examples include calcite , quartz , topaz , beryl , tourmaline and fluorite , among others. Waxy minerals have a lustre resembling wax . Examples include jade and chalcedony . Asterism

2842-511: 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, which appear to move as the specimen is rotated. Such minerals are composed of parallel fibers (or contain fibrous voids or inclusions), 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

2900-723: The most commonly used gemstones in the Late Antique Roman world, and the Migration Period art of the " barbarian " peoples who took over the territory of the Western Roman Empire . They were especially used inlaid in gold cells in the cloisonné technique, a style often just called garnet cloisonné, found from Anglo-Saxon England, as at Sutton Hoo , to the Black Sea . Thousands of Tamraparniyan gold, silver and red garnet shipments were made in

2958-419: The prototype to include chemicals with the general formula A 3 B 2 ( C O 4 ) 3 . Besides silicon, a large number of elements have been put on the C site, including germanium , gallium , aluminum , vanadium and iron . Yttrium aluminium garnet (YAG), Y 3 Al 2 (AlO 4 ) 3 , is used for synthetic gemstones. Due to its fairly high refractive index, YAG was used as a diamond simulant in

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3016-430: The rarest and were first reported in the 1990s. Garnet species' light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives. The mineral's lustre is categorized as vitreous (glass-like) or resinous (amber-like). Garnets are nesosilicates having the general formula X 3 Y 2 ( Si O 4 ) 3 . The X site

3074-534: The seventh century. The brooch was discovered on the North Downs above the village of Kingston, Kent on August 5, 1771 by the Reverend Bryan Faussett (1720-1776), Rector of Kingston. Faussett's excavation of 308 graves revealed an early medieval cemetery. The brooch was uncovered in a burial mound (grave 25) of a small wealthy woman. The grave contained multiple burial goods, including

3132-522: The sharpest edges and therefore performs far better than other kinds of garnet. Both the river and the beach garnet suffer from the tumbling effect of hundreds of thousands of years which rounds off the edges. Gore Mountain Garnet from Warren County, New York , USA, is a significant source of rock garnet for use as an industrial abrasive. Lustre (mineralogy) Lustre ( British English ) or luster ( American English ; see spelling differences )

3190-516: The time describe the brooch as “one of the most curious and, for its size, costly pieces of antiquity ever discovered in England.” A favored decorative gemstone, garnet was used throughout Hellenism, and was particularly prized during the Byzantine period. The ownership of the brooch began with Bryan Faussett. It later passed to his grandson, Godfrey Faussett (1781–1853), Professor of Divinity at

3248-400: The timing of garnet growth. The first dating was that of Basu et al. (1989), who used plagioclase-hornblende-garnet to produce a Sm/Nd isochron that yielded an age of 1059 ± 19 Ma. Mezger et al. (1992) conducted their own Sm/Nd investigation using hornblende and the drilled core of a 50 cm garnet to produce an isochron age of 1051 ± 4 Ma. Connelly (2006) utilized seven different fractions of

3306-532: The yellow crystals resemble, they have also been called hessonite from the Greek meaning inferior. Grossular is found in skarns, contact metamorphosed limestones with vesuvianite , diopside , wollastonite and wernerite . Grossular garnet from Kenya and Tanzania has been called tsavorite. Tsavorite was first described in the 1960s in the Tsavo area of Kenya, from which the gem takes its name. Uvarovite

3364-559: The {100} and {111} families of planes are depleted. Garnets do not have any cleavage planes, so when they fracture under stress, sharp, irregular ( conchoidal ) pieces are formed. Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.0 to 7.5. The harder species like almandine are often used for abrasive purposes. For gem identification purposes,

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