Gemstones Mid Term Study Guide
Gemstones Mid Term Study Guide 1108
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This 11 page Study Guide was uploaded by Caroline Bacevice on Friday March 6, 2015. The Study Guide belongs to 1108 at Ohio State University taught by Loren Babcock in Spring2015. Since its upload, it has received 124 views. For similar materials see Gemstones in Earth Sciences at Ohio State University.
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Date Created: 03/06/15
Gemstones Mid Term Study Guide 0 The Key attribute of a gemstone is Beauty 0 May be in color brilliance transparency re luster or a combination Facet names Table top facet often largest Crown top of the stone bezel top Girdle edge between crown and pavilion Pavilion lower part of a stone about 23 of stone Culet small facet at bottom Gems are faced to take advantage of the behavior of light that passes into a gem Re ection Bouncing of light off a facet mirror effect Refraction Bending of light as it passes from one substance into another Air Gem Refraction Index The degree to which light is bent as it passes from air into a gem This is determined by the density difference between air and gem Placement of each facet is determined using a critical angle which is how the placement of each facet is determined 0 Every stone has one or two refractive indexes 0 Diamonds only have 1 Fire Rainbow like ashes of color in gems Calcite is an example of a great rainbow effectfire What causes re 0 Dispersion rainbow effect degree to which light is refracted which is dependent on wavelength 0 Violet light bends more than red light Some gems are unfaceted not cut Example Pearls Luster Appearance of light as it re ects off the surface of the gem o If the surface is rough some light is lost 0 When the surface is polished light is re ected Durability Hardness resistance to scratching Mohs Scale of Hardness Soft Hard 1 Talc 2 Gypsum 3 Calcite 4 Fluorite 5 Apatite 6 Orthoclase 7Qua z 8 Topaz 9 Corundum 10 Diamond Toughness Resistance to chipping and fracturing Stability Resistance to color loss due to heat light or chemicals The Four C s Cut If a stone diamond is cut to ideal proportions brilliance and re are optimized resulting in greater value 0 Color Some colors are more desirables than others Clarity Flaws inclusions cracks usually decrease value 0 Carat Weight 1 carat is 15 of a gram Larger stones tend to be more valuable Gems formed from Igneous Environments Pegmatite igneous Unusual igneous bodies containing large crystals 0 As magma cools water and rare elements become concentrated in the remaining melt because they do not get to incorporate in the early formed materials 0 When the last of the melt nally crystallizes there is enough water left to allow the last formed crystals to grow large in size and the crystals can incorporate rare elements Hydrothermal deposits igneousrelated Deposits of minerals cooled from superheated water water associated with magma bodies or heated groundwater Gems or metals crystallize from solution in cracks or other open spaces in rocks 0 Veins of minerals ll cracks Gems formed in the mantle igneous 0 Some minerals are formed within the Earth and brought to the surface by volcanic activity Gems in sedimentary environments 0 Sediments or rocks formed by deposition of grains or participation of minerals from water Alluvial deposits 0 Formed from deposition and concentration of mineral grains weathered and eroded from preexisting rock Concentrations usually occur partly because of high density in stream beds and on beaches 0 Placer deposits Alluvial deposits of metals Ceylon Gem Gravels sedimentary deposits 0 IN Sri Lanka gems have been collected for hundreds of years from alluvial deposits 0 Gem weather from igneous or metamorphic rock and then become concentrated in alluvial deposits 0 Similar gem gravels occur elsewhere Precipitation of gems in sedimentary deposits 0 Water near the earth s surface dissolves minerals o If the solution coos or water evaporated new minerals precipitate from solution Gems formed in metamorphic environments 0 Rock and its component mineral are changed by heat and pressure 0 Plate tectonics create metamorphic environments having high temperatures andor pressure 0 Subduction zones 0 Contact metamorphic zones next to cooling magma Crystal a solid having a regular internal arrangement of atoms or ions and at faces geometrically arranged outer surfaces What are crystals made of 0 Elements that compose or give color to most of the important gems 0 Construct Minerals silver aluminum oxygen carbon calcium o Coloration Nitrogen Boron Titanium Beryllium Fluorine Chromium Copper Lithium Vanadium What are the major groups of minerals Silicates sirich minerals o Quartz Beryl tourmaline garnet peridot Oxides O is the anion negatively charged ion 0 Corundum alexandrite Carbonates C and O o Calcite aragonite pearls 0 Elements 0 Diamond C gold Au Silver Ag Platinum Pl Crystals are held together through 0 Atomic bonding o Ions form when atoms gain or lose electrons Anion negatively charged ion Cation positively charged ion Ionic bond Electrons are exchanged Relatively weak bond Salt NaCl Covalent bond electrons are shared Strongest bond Quartz Van der Waals bonds weakest bonds 0 Makes minerals soft or easily breakable along cleavage planes 0 Metallic Bond Electrons are free to travel throughout the substance 0 Fives metals their cohesiveness malleability ductility heat conductivity and electrical conductivity Polymorphs Minerals having the same chemical composition but different crystal structures Crystal Symmetry Symmetry Faces on a crystal have a symmetry because of the arrangement of atoms 0 Important for 0 Identifying minerals in uncut form 0 Determining the best way to cu a crystal Crystal Systems 0 Cubic At least four threefold axes of symmetry 0 Diamond or Garnet gold silver platinum Hexagonal threefold or sixfold symmetry 0 Beryl sapphire ruby emerald aquamarine quartz Tetragonal One fourfold axis of symmetry 0 Zircon o Orthorhombic At least three twofold axes of symmetry 0 Topaz o Monoclinic At least one twofold axis of symmetry 0 Orthoclase Feldspar o Triclinic no axis of symmetry 0 Amblygonite Diamond Name From Old French diamant after Greek and Latin adamas meaning invincible or quotunconquerablequot Characteristics of Diamonds 0 Composition Pure Carbon C Hardness 10 Streak White 0 Speci c gravity 352 o Refraction lndex 242 dispersion 0044 Crystals cubic system often octahedrons or cubes Luster Adamanite Formation of Diamonds 0 From near the coremantle boundary 0 Depth 2900 km 1800 miles 0 High pressure conditions 9001300 degrees C 45 to 60 kilobars Diamonds are carried to the surface as inclusions in pipes of volcanic rock and erupted o Kimberlite or lamproite peridotite an ultrama c rock Graphite not diamond is the stable form of Carbon at the Earth s surface 0 Diamond will burn or oxidize on the surface if dropped in a re for a short time 0 Both diamond and graphite are polymorphs of Carbon 0 Diamond atoms are linked in a 3D network 0 Graphite atoms are linked in sheets Ancient times to at least 800 BC all diamonds came from alluvial gravels in India eroded from kimberlite piped in the Golconda area 1725 Diamonds were discovered in alluvial gravels of Brazil Continues to be major source of diamonds today The South African diamond rush 1859 Diamonds discovered in alluvial gravel rivers of South Africa 1867 First authenticated nd 1870 Diamonds discovered in kimberlite pipes quotblue groundquot or quotyellow groundquot if weathered near Kimberly 1871 Diamonds discovered on the de Beers farm resulting in a large mine called the quotBig Holequot There are 5 carats in a gram Until recently Africa accounted for 98 of diamond production 0 Africa now accounts for lt50 Australia lamproite pipe and Russia kimberlite pipe are now large diamond producers Other sources Venezuela China Canada and USA 0 American diamonds Occasionally found in gravels some are in ultrama c pipes De Beers 1870 Cecil Rhodes arrived in Durban South Africa from England 1873 Bought a claim in the de Beers mine at Kimberley South Africa 1880 Rhodes formed the de Beers Mining Co Ltd 1888 Rhodes formed De Beers Consolidated Mines Ltd To control diamond mining and control output 0 Being limiting sales of diamonds to stabilize prices 0 Establish a syndicate of diamond buyers in Kimberley and London 0 Formalized the process of equating the supply to demand 0 Syndicated marketing policy was largely successful until increasing in 1890 Rhodes elected Prime Minister of South Africa died in 1902 1902 Ernest Oppenheimer arrived in Kimberley as a diamond buyer 1917 Oppenheimer set up AngloAmerican Corporation to exploit gold elds in the East Rand South Africa Became Chairman of De Beers 1929 o In the mid 19205 new diamond discoveries in western Africa plus high production costs at Kimberley caused falling prices 0 1929 Great Depression began 0 Diamond market collapsed syndicate had deep nancial problems 0 Oppenheimer formed the Diamond Corporation to help stabilize prices through a single sales channel 0 1933 De Beers could not cover operating expenses closed mines at Kimberley in 1934 0 To counteract these conditions Oppenheimer persuaded producers in all countries that one production and sales channel would cushion the diamond trade against boom and bust cycles 0 De Beers in cooperation with South African government led producers to form the Diamond Producers Association DPA 1957 Oppenheimer died De Beers Production and marketing strategy DPA Formulates policy and sets quota o Comprises De Beers in South Africa and Namibia Diamond Corporation a de Beers subsidiary government of South Africa which owns mines o DPA makes marketing agreements with outside producers through Diamond Corporation 0 Central Selling Organization CSO Group of marketing companies diamond producers cooperatively sell diamonds 0 Diamonds are rst sent to DPA Johannesburg or the Diamond Corporation 0 Sorted into gem and industrial quality stones and then sold through different organizations 0 Rough gem diamonds are sold via the Diamond Purchasing and Trading Company to the Diamond Trading Company 0 Parcels of xed prices are offered to buyers at 10 sales sights per year 0 Rough diamonds then make their way to gem cutters The CSO controls 6080 of the worlds legal trade in diamonds 0 Prices are controlled by stockpiling goods when demand is low forcing prices to remain high and incrementally increasing the price when demand is high De Beers was for a long time been prohibited from directly selling diamonds in the USA by the Sherman Antitrust Act What determines the value of a diamond Cut Faceted to maximize the brilliance and re of a gem o Poorly cut stone has poorly chosen proportions poor brilliance poor re light leakage from pavilion misplaced facets extra facets etc o A Natural an extra face that represents a polished extra face of a crystal Fancy colored stones 0 Large pink diamonds rare and expensive 0 Natural blue diamonds contain boron natural yellow diamonds contain nitrogen Standards 0 International standard 1970 RAL 560 ASE o Gemological Institute of America GIA 0 International Diamond Council IDC Clarity o Clarity decreased by aws or blemishes o Scratches o Inclusions minerals liquids gasses contained in the stone 0 Carat weight 0 1 carat 02 g o 1 carat 100 points Treatment of Diamonds 0 Filling of cracks Cracks that reach the surface are lled with glasslike material 0 ID Optical microscope examination greasy appearance ash effect bubbles Drilling of inclusions by laser Solutions poured into inclusions to bleach them Irradiation Used to change the color 0 ID Irradiation only changes the surface of the stone and produce a concentration of color where the gem is thin Industrial Diamonds 0 Bort Poorly crystallized black gray or brown translucent to opaque Ballas spherical aggregates of many small diamond crystals Carbonado Opaque black or gray tough and compact o Meteorites 0 Synthetic Diamonds Famous Diamonds 0 Cullinan I Star of Africa 0 Cullian II now in English Imperial State Crown KohiNoor Mountain of Light VictoriaTransvaal Diamond Tarzan movie MarieAntoinette s diamond earrings Napoleon Necklace Hope Diamond 0 Now in the Smithsonian Institution Viewed by more than gt6 million visitors annually Probably the most sought out object in any museum in the world 0 Why Now the world s largest diamond 4552 carats Largest cut diamond Golden Jubilee owned by the King of Thailand It s deeply colored doesn t seem like a typical diamond o It is the largest known deep blue diamond Blue color one of the rarest colors of fancy colored diamonds Due to substitutions of Boron for carbon 1 B atom per 1000000 C atoms 0 History gt300 years of mystery and intrigue o Discovered in India 0 1668 JeanBaptiste Tavernier French gem merchant sold a 1105 carat blue diamond from India to King Louis XIV of France Cut in Indian stylenatural crystal faces were polished size emphasized over brilliance o 1673 King Louis XIV had stone recut to heart shape improving brilliance reducing size to 6903 carats o 1749 King Louis XV had the stone now called French Blue set in a piece of ceremonial jewelry Order of the Golden Fleece worn by the Queen 0 1792 French Revolution erupted during the reign of Louis XVI and Queen MarieAntoinette The royal treasury and the Crown Jewels were looted between September 11 and September 17 1792 French Blue disappeared for 20 years 0 By French Law the statue of limitations on wartime crimes was 20 years 0 On September 19 1812 20 years and 2 days after the theft of the French Crown Jewels a London jeweler documented a 455 carat blue diamond in England in the possession of Daniel Eliason a London diamond merchant Memo established Eliason as the new legal owner The diamond was apparently cut from French Blue About 1820 King George IV of England purchased the diamond After George lV s death in 1830 Lord Henry Philip Hope a London banker bought the diamond Hope had the diamond mounted in a medallion Passed through Hope family until 1901 1910 Hope Diamond sold to CH Rosenau then Pierre Cartier Cartier reset the diamond and sold it to Evalyn Walsh McLean for 180000 Cartier told McLean that the diamond brought bad luck to anyone who wore it Apparent origin of the supposed curse it was a sales pitch McLean thought quotbad luckquot objects brought her goodluck McLean added to the legend of the Hope Diamond Wore it frequently allowed others to wear it Pawned it several times to raise money 1949 two years after McLean s death Harry Winston NYjeweler purchased McLean s jewelry collection 0 The diamond traveled in an exhibit 1958 Hope Diamond was removed from its setting culet was recut slightly November 1958 Winston donated the Hope Diamond to the Smithsonian Institution USNM as the foundation for a National Gem Collection Color perceived depends on the light under which an object is viewed Different types of light different color perceptions eg incandescent light vs uorescent light vs ultraviolet light 0 O 0 If atoms in a gem absorb all red light only greenblue light will be transmitted to the eye stone will appear green blue Black all colors absorbed White or colorless No light is absorbed Origin of color can be 0 O 0 Chemical Physical Optical Body color color resulting from internal chemical composition Idiochromatic gems quotSelfColoredquot due to colorcausing elements that are part of the chemical composition 0 Ex Rhodolite garnet Allochromatic gems quotOther cooredquot due to impurities Fe Mn Cu Ni Co Ti V Cr etc 0 Ex Corundum lt1 Cr changes a colorless stone to a ruby Oxidation state also affects color 0 In beryl Fe2 yields blue bery aquamarine Fe3 Yellow beryl heliodor Mn2 Pink Beryl morganite Mn3 Red beryl bixbite o In aochromatic gems the color can be changed if the oxidation state has changed Yellow bery can be heated in a low oxygen environment reducing ferric iron Fe3 to ferrous iron Fe2 and producing blue bery
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