Gemstones Final Study Guide
Gemstones Final Study Guide 1108
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This 21 page Study Guide was uploaded by Caroline Bacevice on Saturday April 25, 2015. The Study Guide belongs to 1108 at Ohio State University taught by Loren Babcock in Spring2015. Since its upload, it has received 87 views. For similar materials see Gemstones in Earth Sciences at Ohio State University.
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Date Created: 04/25/15
Gemstones Final Study Guide The Key attribute of a gemstone is Beauty 0 May be in color brilliance transparency fire 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 9 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 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 fire 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 If the surface is rough some light is lost When the surface is polished light is re ected Durability Hardness resistance to scratching Mohs Scale of Hardness Soft 9 Hard 1 Talc 2 Gypsum 3 Calcite 4 Fluorite 5 Apatite 6 Orthoclase 7 Quartz 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 fire are optimized resulting in greater value Color Some colors are more desirables than others Clarity Flaws inclusions cracks usually decrease value 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 finally 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 fill cracks Gems formed in the mantle igneous Some minerals are formed within the Earth and brought to the surface by volcanic activity Gems in sedimentary environments 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 IN Sri Lanka gems have been collected for hundreds of years from alluvial deposits Gem weather from igneous or metamorphic rock and then become concentrated in alluvial deposits Similar gem gravels occur elsewhere Precipitation of gems in sedimentary deposits Water near the earth s surface dissolves minerals If the solution cools or water evaporated new minerals precipitate from solution Gems formed in metamorphic environments Rock and its component mineral are changed by heat and pressure Plate tectonics create metamorphic environments having high temperatures and or 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 Elements that compose or give color to most of the important gems 0 Construct Minerals silver aluminum oxygen carbon calcium 0 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 Elements 0 Diamond C gold Au Silver Ag Platinum Pl Crystals are held together through Atomic bonding o Ions form when atoms gain or lose electrons I Anion negatively charged ion I Cation positively charged ion Ionic bond Electrons are exchanged I Relatively weak bond I Salt NaCl Covalent bond electrons are shared I Strongest bond I Quartz Van der Waals bonds weakest bonds 0 Makes minerals soft or easily breakable along cleavage planes 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 Important for 0 Identifying minerals in uncut form 0 Determining the best way to cu a crystal Crystal Systems 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 Orthorhombic At least three twofold axes of symmetry 0 Topaz Monoclinic At least one twofold axis of symmetry 0 Orthoclase Feldspar Triclinic no axis of symmetry 0 Amblygonite Diamond Name From Old French diamant after Greek and Latin adamas meaning invincible or unconquerable Characteristics of Diamonds Composition Pure Carbon C Hardness 10 Streak White Specific gravity 352 Refraction Index 242 dispersion 0044 Crystals cubic system often octahedrons or cubes Luster Adamanite Formation of Diamonds 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 ultramafic rock Graphite not diamond is the stable form of Carbon at the Earth s surface Diamond will burn or oxidize on the surface if dropped in a fire for a short time 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 find 1870 Diamonds discovered in kimberlite pipes blue groundquot or yellow groundquot if weathered near Kimberly 1871 Diamonds discovered on the de Beers farm resulting in a large mine called the Big 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 ultramafic 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 fields in the East Rand South Africa Became Chairman of De Beers 1929 In the mid 1920s new diamond discoveries in western Africa plus high production costs at Kimberley caused falling prices 1929 Great Depression began 0 Diamond market collapsed syndicate had deep financial 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 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 Central Selling Organization CSO Group of marketing companies diamond producers cooperatively sell diamonds 0 Diamonds are first sent to DPA Johannesburg or the Diamond Corporation 0 Sorted into gem and industrial quality stones and then sold through different organizations Rough gem diamonds are sold Via the Diamond Purchasing and Trading Company to the Diamond Trading Company 0 Parcels of fixed 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 fire of a gem o Poorly cut stone has poorly chosen proportions poor brilliance poor fire 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 Carat weight 0 1 carat 02 g o 1 carat 100 points Treatment of Diamonds Filling of cracks Cracks that reach the surface are filled 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 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 Cullinan I Star of Africa Cullian 11 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 I Viewed by more than gt6 million Visitors annually I Probably the most sought out object in any museum in the world I Now the world s largest diamond 4552 carats I Largest cut diamond Golden Jubilee owned by the King of Tha and I It s deeply colored doesn t seem like a typical diamond o It is the largest known deep blue diamond I 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 Discovered in India 0 1668 leanBaptiste Tavernier French gem merchant sold a 1105 carat blue diamond from India to King Louis XIV of France I 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 O 1792 French Revolution erupted during the reign of Louis XVI and Queen MarieAntoinette I The royal treasury and the Crown Iewels were looted between September 11 and September 17 1792 French Blue disappeared for 20 years By French Law the statue of limitations on wartime crimes was 20 years 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 I Memo established Eliason as the new legal owner I The diamond was apparently cut from French Blue About 1820 King George IV of England purchased the diamond After George IV s death in 1830 Lord Henry Philip Hope a London banker bought the diamond I 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 I Cartier told McLean that the diamond brought bad luck to anyone who wore it I Apparent origin of the supposed curse it was a sales pitch McLean thought bad luckquot objects brought her good luck McLean added to the legend of the Hope Diamond I Wore it frequently allowed others to wear it I Pawned it several times to raise money 1949 two years after McLean s death I Harry Winston NY jeweler purchased McLean s jewelry collection 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 If atoms in a gem absorb all red light only greenblue light will be transmitted to the eye stone will appear greenblue Black all colors absorbed White or colorless No light is absorbed Origin of color can be 0 Chemical 0 Physical 0 Optical Body color color resulting from internal chemical composition Idiochromatic gems SelfColoredquot due to colorcausing elements that are part of the chemical composition 0 Ex Rhodolite garnet Allochromatic gems Other coloredquot due to impurities Fe Mn Cu Ni Co Ti V Cr etc 0 Ex Corundum I lt1 Cr changes a colorless stone to a ruby Oxidation state also affects color 0 In beryl I Fe2 yields blue beryl aquamarine I Fe3 Yellow beryl heliodor I Mn2 Pink Beryl morganite I Mn3 Red beryl bixbite o In allochromatic gems the color can be changed if the oxidation state haschanged I Yellow beryl can be heated in a low oxygen environment reducing ferric iron Fe3 to ferrous iron Fe2 and producing blue beryl Colors created Chemically The same impurities can color different gems differently o Chromium Cr3 I In corundum produces red ruby I In beryl produces emerald green emerald Charge transfer when electrons are swapped between elements through heating 0 Elements that oftenhave change transfer I Fe2Fe3 I Ti3 Ti4 I Mn2 Mn3 Mn4 In sapphire heating transfers Fe2 and Ti4 Phenomena Colors resulting from the interaction of light with physical structure of a stone Iridescence Play of colors Pleochroism Color change in different lights Color Centers Structural imperfections in crystals causing selective absorption of light Pleochroism Different colors that gems appear to have when viewed under different lights or in different directions 0 Caused by differing absorption of light rays in doubly refractive crystals Corundum Sapphire and Ruby Origin and History of names 0 Corundum from Iauruntaka the Indian name for the mineral 0 Ruby from Latin ruber meaning red I Until about 1800 rubies were not recognized a variety of corundum I Red garnet and red spinel were also once called rubies 0 Sapphire from Latin sapphirus meaning blue 0 From antiquity through the Middle Ages the word sapphire was applied to lapis lazuli Origin of Rubies and Sapphire Rubies 0 Primary source metamorphic rock dolomitic marble gneiss amphibolite 0 Secondary source Alluvial deposits more economically viable 0 Burma Myanmar Thailand Sri Lanka Tanzania etc Sapphires 0 Primary sources Metamorphic rock dolomitic marble igneous rick basalt pegmatite 0 Secondary source Alluvial deposits 0 Sri Lanka Myanmar Australia Thailand Brazil etc Inclusions Inclusion Mineral gas or uid contained within a crystal In transparent stones inclusions are undesirable resulted n awed stones In opaque stones some inclusions are desirable and add value 0 Asterism Silklike texture caused by needles of rutile TiOZ o Rutile grows parallel to hexagonal faces of corundum 60 between needles 0 Cutting corundum perpendicular to crystal axis results in star pattern Synthetic Corundum Rubies and sapphires have been produced in the laboratory by ame fusion Verneuil process since 1908 0 Metallic oxides are added to colorless corundum o Boule synthetic crystal formed by ame fusion Also produced by ux growth Linde stars synthetic star sapphires and rubies introduced by Linde Corp 1947 0 Titanium oxide added to ame fusion mix to produce rutile crystals Famous Stones Star of Bombay deep blue star sapphire 182 carats 0 From Sri Lanka now in the Smithsonian 0 Once owned by movie star Mary Pickford given by husband Douglas Fairbanks Ir DeLong Star Ruby 1003 carats 0 From Burma Myanmar 1930s donated to the American Museum of Natural History in 1838 stolen in The Great Iewelry Robberyquot 1964 Ransomed 25000 Star of India light blue sapphire 56335 carat 0 From Sri Lanka pre 1900 donated to AMNH by JP Morgan Star of Asia 330 carats large intense color sharp star 0 From Burma now in USNM Carmen Lucia Ruby 0 Exceptionally large and clear ruby 231 carats Mined in Burma in 1930s Set in platinum ring Purchased from a New York jeweler buy Peter Buck founder of subway donated to Smithsonian in the name of his late wife Carmen Lucia Buck O O O 0 Industrial Uses for Corundum Sandpaper emery boards nail files Lasers Beryl Beryl From ancient Greek beryllos originally applied to all green stones Emerald From Greek smaragdos green Aquamarine From Latin aqua marina sea water Heliodor golden From Greek Helios Sun Morganite pink Violet peach salmon After JP Morgan American financier Goshenite colorless After Goshen Massachusetts The three main sources of mineral or gem names are from color a person or a locality Origins of Beryl Beryl Usually occurs in veins or pegmatites igneous rocks Emerald also occurs as hydrothermal grown in limestone in mica schist and gneiss metamorphic Secondary sources Alluvial gravels Egypt ancient source of emeralds Colombia Brazil Russia Africa Maine North Carolina Utah etc Synthetic Emeralds Grown in laboratory by ux method Chatham since 1940 Gilson or by hydrothermal method Linde Synthetic emeralds lack many inclusions have lower RI and SG than natural stones etc Famous Emeralds Hooker Emerald 7547 carats square 27 mm on each edge 0 Found in Colombia shipped to Europe by Spanish conquistadores ca 1700s 0 Owned by Abdul Hamid II Sultan ofthe Ottoman Empire 1861909 0 Acquired by Tiffany amp Co before World War I o Purchased by Janet Annenberg Hooker 1955 gift to USNM 1977 Spanish Inquisition Necklace 0 Composed of large emeralds and diamonds fashioned in India in the 1600s Two strands of antiquecut emeralds 15 and diamonds 374 Largest emerald is 45 carats Emeralds from Colombia diamonds from India Once the property of Spanish Royalty later ladies of the French court Purchased in early 1900s by the Maharaja of Indore Acquired by Harry Winston from the Maharaja s son in 1948 Sold to Cora Hubbard Williams 1955 bequeathed to USNM in 1972 The MariwLouise Diadem 0000000 0 Gift of Napoleon Bonaparte to Empress Marie Louise in 1810 0 Originally set with 70 emeralds and 1006 mine cut diamonds in silver and gold 0 Part of a set that included a necklace comb belt buckle earrings 0 Set passed to Hapsburg family 0 Purchased by Van Cleef amp Arpels 1953 o Emeralds removed and sold in other pieces of jewelry replaced with turquoise o Purchased for USNM by Marjorie Merriweather Post 1971 Topaz Origin and History of Name Topaz Possibly after Topazos in the Red Sea now called Zabargad qupt 0 Topazos was the ancient source of peridot In ancient times all yellow and man greens and golden brown gems were called topaz Sometimes called precious topaz or imperial topaz to distinguish it from quarts varieties eg lemon topaz citrine smoky topazsmoky quartz Irradiating Topaz Topaz is the most commonly irradiated gem on the market today However irradiation is used to change the color of many gems Radiation changes the natural color of the gem reducing the value Irradiation cannot be detected Color is not always stable Safety Residual radioactivity is potentially a problem 0 Stone may need to be stored to allow radioactivity to decay halflife may be hours to delay Zircon Origin and History of Name Zircon Origin of name is uncertain Possibly from Arabic zargun derived from Persain zar and gun gold colored Origin of Zircon Zircons are usually found in gravel deposits 0 Zircon originally forms as small crystals in igneous and metamorphic rocks but these sources are not mined o Zircon is highly resistant to weathering and has a high SG both of which favor its concentration in alluvial gravels Most important localities Laos Cambodia Thailand Myanmar Sri Lanka Important Characteristics of Zircon Double refraction of crystals causing obvious doubling of facets Contains uranium U and Thorium Th both of which undergo radioactive decay to lead Pb o Crystals become damaged cloudy with radioactive decay metamict Zircons are used for isotopic age dating Treatment of Zircon Crystals that have become cloudy due to radioactive decay Metamict crystals can be restored by heat treatment 1450 Celsius Heating of brown Zircon 8001000 Celsius o In reducing environment Produces colorless or blue zircon o In air produces colorless or yellow zircon Most colorless or blue stones have been heat treated often brilliant cut to resemble diamonds o Distinguished from diamonds by double refraction Heating makes zircon more brittle and faces abrade more easily Chrysoberyl From Greek chrysos golden and beryllos referring to beryl Golden beryl is a misnomer Alexandrite variety showing color change in different types of light Named for Czar Alexander 11 of Russia Cymophane From Greek cymos waving and phanos light Origins Occurs in igneous rocks granites pegmatites and metamorphic rocks mica schists Also occurs in alluvial deposits in Sri Lanka Brazil China India Alexandrite Discovered in the Ural Mountains Russia in 1833 Now found in gem gravels of Sri Lanka mined in Zimbabwe Brazil Myanmar Zambia India Synthesized by ame fusion Czochralski processes Much labcreated alexandrite is berylliumOdoped corundum sapphire Chatoyancy Chatoyancy Silky sheen resulting from closely packed parallel fibers or parallel needlelike inclusions CAT S EYE EFFECT Produces cat s eye effect when stones are cut en cabochon rounded Chatoyancy is present in various minerals chrysoberyl quarts etc o Chatoyant chrysoberyl is sometimes called cymophane o Tiger s Eye quarts replacement of fibrous crocidolite asbestos Spinel From latin spina little thorn referring to sharp points on octahedral crystals Redspinel has often been confused with ruby 0 Many historic quotrubiesquot were probably spinel Origin Igneous and metamorphic mineral Gem gravels of Burma Sri Lanka Thailand associated with corundum Synthesized by ame fusion Verneuil method Often used as a cap on doublets or triplets Famous Spinel Black Prince s Ruby In the English Imperial Crown of State 0 Once thought to be a ruby o Specimen is 5 cm long uncut just polished Quartz Transparent Varieties Rock Crystal Colorless Amethyst Purple Citrine Yellow Smoky quarts brown gray or black Rose quartz Pine Ametrine Colorzoned amethyst citrine Rutilated quartz Colorless usually with inclusions of rutile needles Prase prasiolite Leekgreen Quartz Origin and history of some names Quartz From German quartz originally applied to massive vein quartz applied to all varieties by the end of 18th century Rock crystal from Greek krystallos ice Amethyst From Greek amethystos not to intoxicate ancient belief was that wine drunk from an amethyst cup would not cause intoxication Citrine From old French Citron yellow Chalcedony Possibly from Chalcedon an ancient seaport in Asia Minor Carnelian From Latin carnis esh referring to the red color Prase From Greek prason leek in allusion to its green color Origin of Quarts Common in most igneous sedimentary and metamorphic rock Quartz is quite resistant to mechanical abrasion it is the most abundant mineral following weathering and erosion 0 Common in stream sediments on beaches in windblown deposits Synthetic quartz First synthesized in the 1800s by the hydrothermal method Heat Treatment of Quartz Formation of citrine from amethyst is common 0 Often done by using a wheelbarrow to immerse crystals in sand within a fire 0 Crystals are heated to a high enough temperature to change the oxidation state of iron in the mineral Piezoelectricity little electricity What is piezoelectricity o A few crystal types have polar axes instead of a symmetry center 0 The opposite axes have different properties 0 Pressure exerted at the ends of a polar axis causes electrons to ow to one end producing a negative charge and a positive charge is induced at the other end Quartz and tourmaline are strongly piezoelectric Plates cut from quartz have been used as oscillators to control radio frequency 0 When subjected to alternating current of a radio circuit quartz vibrates at a frequency that depends on plate thickness and type of cut Tiny quartz plates in watches vibrate at a constant predetermined frequency to control the radio frequency of an electric circuit that provides the time display 0 Synthetic quartz is used today for this purpose Quartz Translucent to Opaque Varieties Chalcedony Cryptocrystalline quartz o Agate Banded all colors Petrified Wood wood fossilized by quartz Crysoprase Green Bloodstone Dark green with red spots Iasper All colors often reddish brown Carnelian Brownish red to orange Moss Agate Colorless with green brown or red inclusions of hornblende or chlorite in mosslike patterns 0 Onyx Layered commonly black base and white upper layer 0 Sard red brown to brown Flint Fine grained quartz Tiger s eye quartz Chatoyant with asbestos needles gold yellow and gold brown Types of Opal Precious opal has distinctive play of colors opalescence 0 Play of colors due to microscopic spheres of cristobalite silica layered in siliceous gel 0 Cristobalite spheres cause diffraction and interference patterns Fire Opal yellowred or orange color often without play of color Transparent best to translucent Common opal Opaque rarely translucent without play of colors OOOOOO Water in Opal Treatments Opal always contains some water 0 Typically 330 water In time the stone may lose water causing cracking and reduction of opalescence 0 Can be restored temporarily by saturation with oil epoxy resin or water 0 Storing opal in moist cotton minimized dehydration Care is needed in mounting opal in jewelry Heat used in soldering can evaporate the water Opal is often impregnated with plastic to improve its appearance Often mounted with quartz glass or spinel cap doublet Occurrences Chalcedony and fine grained quartz occurs in igneous and sedimentary rocks worldwide Opal occurs in sedimentary rocks in many areas of the world 0 Major sources Australia Mexico Obsidian Occurs in igneous rocks in many parts of the world 0 Major sources SW USA Tourmaline Group Tourmaline From Sinhalese turmali stone with mixed colors 0 Shown well in bicolor tourmaline or watermelon tourmaline color zoned pink center and green rim Occurs in igneous rocks especially pegmatites metamorphic rocks and gem gravels Garnet Group Garnet From Latin granatus grain Pyrope red From Greek pyros fiery Tsavorite green grossular From Tsavo National Park Kenya Occurs in igneous metamorphic and sedimentary rocks Also in alluvial deposit Found Worldwide Lapis Lazuli Origin of name From Persian Iazhward blue Composition A rock not mineral composed of lazurite calcite pyrite commonly sodalite and other minerals Origin metamorphic Color Blue mixed with white and brassy speckles Turquoise Origin of name From French turquoise Turkish Turkish Stone because the trade route that brought the gem to Europe passed through Turkey Composition Hydrated copper aluminum phosphate Origin Fills fissures in weathered volcanic rocks Colors Medium blue greenishblue green Gold Au Symbol for Latin work aurum Silver Symbol from Latin argentum Distinguished characteristics develops a gray to black tarnish Both occur in hydrothermal veins commonly with quartz pyrite etc Second source Placer deposits akes nuggets Pyrite Fool s Gold Distinguished by 0 Greater hardness o Brownish black or greenish black streak Brassy mineral disseminated in lapis lazuli Platinum From Spanish platina silverlike Distinguishing Characteristics contains up to 28 iron distinctly magnetic Alloys Precious metals in the pure state are soft To increase hardness and durability alloys mixtures of the metals are used Alloys can be used to change the color of gold white rose green Gold Alloys 0 Karat K or KT Amount of pure gold present in the alloy American usage 0 Maximum 24K 0 European marking based on portions of 1000 parts pure gold1000 Other Markings Gold Plate marked GP EP Thin layer of gold electroplated to surface of a base metal 0 Gold is easily worn off because it is only a few microns thick Sterling Silver 925 pure silver 925 sterling Silver Plating Silver electroplated over another metal alloy Platinum usually marked as PLAT or 950 Pearl Origin Formed by mollusks clams especially oysters snails Pearl oysters secrete alternating layers of aragonite CaCO3 86 and conchiolin organic 10 Concentric mineral layers are deposited around an irritant o Parasite or other irritant Iridescence orient of pearls is due to overlapping plates of nacre mother of pearl Nacre Alternating layers of aragonite and conchiolin Cultured Pearls A small bead of shell is wrapped in a piece of mantle tissue Bead is inserted into the oyster next to inner side of shell 0 Bead irritates oyster o Oyster deposits nacre over it Oysters are returned to the water in cages and harvested after a couple of years 0 2 years needed to deposit 1 mm layer of nacre Pearl Valuation Valued according to shape color size surface condition and luster Nearly spherical large nearly awless and lustrous pearls are more highly valued Baroque pearls pearls having unusual shapes 0 Example Hope Pearl owned by Lord Hope same as Hope diamond History of Pearls Used as gems for gt6000 years Pearl trade in China by 2500 BC Cultured Pearls produced in China by the 1200s Round cultured pearls first produced by Carl von Linne Carolus Linnaeus in 1761 in river mussels of Sweden Kokichi Mikimoto first produced cultured pearls on a large commercial scale in 192 1 Simulants glass plastic Amber Origin Hardened tree resin in sedimentary deposits Resins are produced by conifer trees as a defense against insects and disease Amber Fossil resin gt11700 to million years old Copal Subfossil Resin Amber Characteristics Composition mix of organic resins Hardness 225 Colors mostly yellow brown sometimes greenish Streak white SC 108 Floats in salt water RI 154 SR Luster Vireous to resinous polished Other characteristics Amorphous Inclusions are common Occurence of Amber Largest deposit blue clay near Kaliningrad Russia Baltic Sea area washes up onto shore Collected as far away as Dominican Republic Historical Uses of Amber Used for adornment since at least the Paleolithic age Used for jewelry and artwork by ancience Greeks Romans and Egyptians Used in Europe during Middle Ages for religious objects and Secular art Ancient Egypt resins used as a base for paint Varnish and lacquer have long been produced from ground resins Shell essentially same as Pearl Origin Aqueous mollusks Composition Calcium carbonate Oldest known jewelry pierced snail shells 82000 years old from Morocco Origin Skeletons of marine corals tropical seas Composition Aragonite Hardness 34 Colors Various Streak White Luster Vitreous polished Origin Elephant Tusk mammoth tusk walrus tusk etc Composition Calcium Phosphate Hardness 23 Color White cream Streak white Luster Pearly Iet Origin Bituminous coal sedimentary deposits worldwide but mostly from England Composition Carbon Hardness 254 Colors Black dark brown Streak Black Natural and Imitation jet 0 Whitby jet natural coal 0 French jet black glass
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