Lecture 13 Notes (Geoscience 331)
Lecture 13 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
Popular in Geology
This 0 page Class Notes was uploaded by Hannah James on Saturday November 28, 2015. The Class Notes belongs to Geoscience 331 at University of Wisconsin - Madison taught by Huifang Xu in Fall 2015. Since its upload, it has received 16 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 11/28/15
Organic Gem Materials Lecture 13 Pearls In mineralogy minerals are crystalline substances produced by inorganic processes 0 That is to say not formed by living organisms 0 There are a few organic materials that have been used as gems for thousands of years Physical Characteristics 0 Hardness 25 45 O SG 270 freshwater up to 274 0 Size from microscopic to many centimeter diameter rare 0 Luster typical pearly luster is termed quotorientquot Composition 0 86 calcium carbonate CaCO3 mineral aragonite high density form of CaCO3 weight 0 24 water 0 10 conchiolin an organic binding agent The color of a pearl depends upon the type of mollusk and the water composition 0 Polluted water can produce unusual coloration 0 Color is both a function of the nacre and any impurities taken from the water such as transition metals organic materials pollutants etc Two primary colors of pearls 0 Body color I This is the underlying color and is usually a whiteyellow cream or black 0 Overtone I Sometimes described as quot oatquot the overtone resembles a filmy lacquer and can be many colors such as pink green or blue 0 Many colored pearls have a creamwhite body color and overtones of another color pink is quite popular Nacre consists of a series of alternating layers of conchiolin and crystals of aragonite CaCO3 O The CaCO3 is in the crystal form known as aragonite instead of calcite a low density form 0 The typical iridescence of the pearl is due to the series of overlapping nacre layers I This is referred to as 39orient39 iridescent effect due to overlapping nacreous plates in semiperiodic way 0 Only pearls from mollusks with a nacreous mother of pearl lining are quottruequot pearls Formation O Pearls are produced by a variety of organisms commonly marine mollusks I They are also produced by fresh water mussels and occasionally by snails Natural Pearls O Pearls are calcareous concretions of concentric layers of CaCO3 deposited around an irritant I This may be a piece of mantle lobe or some other material 0 Only the mollusk39s mantle lobe part of the tissue connected to the shell can secrete nacre I When a piece of mantle lobe is introduced by some accident into the internal tissue of the oyster the oyster forms a bag known as a quotpearl sac 0 It is this sac that secretes the nacre around the irritant to make the pearl I Some natural pearls have quite unusual shapes 0 These are often called baroque pearls 0 Both saltwater and freshwater pearls consist of the same material and can form in quotbaroquequot shapes I Unless you are familiar with the typical characteristics of pearls from specific sources it would be very difficult to know whether it had a saltwater or freshwater in origin I Probably the most common freshwater pearl on the market is the Chinese freshwater baroque some of which are crinkly and look like crisped rice 0 These have been popular recently because they cost less than Akoya cultured pearls 0 Blister Pearls O Blister pearls form on the inside of the mother of pearl shell and have a halfround shape I Cultured Pearls O Oysters and mussels can be induced to make pearls by placing a bead referred to as a quotseedquot of shell into it I The resulting pearl is termed a quotcultured pearlquot 0 Nearly 90 of the pearls sold are cultured natural pearls have become quite rare I This is partly due to the fact that harvesting natural pearls quickly destroys large communities of mollusks O Culturing pearls is like farming crops of pearl oysters are suspended in baskets all of them putting layers of nacre over a seeded bit of shell I If you break a pearl open you will see that it consists of a bead covered by a thin layer of nacre O The culturing process involves inserting a small piece of mantle lobe and a bead made from mother of pearl shell into the tissues of a pearlproducing mollusk I The mollusk treats the bead as an irritant and the mantle lobe tissue begins to deposit a nacreous coating over it I The commercial production method for cultured pearls is known as the MiseNishikawa method 0 Oysters are raised in a tank allowed to attach to fibers then grown in sea water for two to three years I Growing oysters are suspended in cages hung from rafts 0 They feed on plankton 0 Healthy oysters are selected for pearl cultivation I The bead is prepared using quotPig Toequot clam shells from the Mississippi River pearliculture industries throughout the world use these clams 0 Small balls are prepared from pieces of these shells 0 Living oysters are wedged open and a piece of mantle tissue harvested from another oyster plus a bead are inserted into the soft tissue 0 Oysters are then returned to the sea where they are suspended in cages 7 10 feet below the surface I The culture period used to be 35 yrs 0 Over that time a 1mm layer of nacre will deposit on the bead 0 Demand for cultured pearls has resulted in a shortening of the process and pearls may take less than 2 yrs depositing a proportionally thinner layer of nacre Typical results from pearliculture O 28 marketable pearls only 5 are high quality pearls hanadama O 17 unmarketable pearls O 5 uncoated nuclei 0 50 of oysters containing nuclei will die Pearls are selected for their size and color hue O Careful color grading is extremely important 0 Pearls are then drilled from both sides often at a place that is slightly awed I They are then sorted drilled and threaded with string and marketed Mabe pearls O Mabe pearls are cultured blister pearls I These are produced by inserting a half bead against the shell of the mollusk I After a layer of nacre has been deposited over the bead the whole formation is cut out and the nacreous dome cemented onto a mother of pearl bed Biwa pearls O Biwa pearls are produced at lake Biwa Japan using freshwater clams I They are irregular in shape but have good color and luster Natural or Cultured O Distinction between these can often be made if the pearl is drilled because the size and nature of the seed can be determined 0 For cultured pearls you should see a mm or so beneath the surface 0 The term quotcandlingquot refers to examination of pearls in strong light I This may reveal the mother of pearl bead I If the pearl is undrilled an excellent method to distinguish cultured from natural pearls is to Xray them 0 Natural pearls are less dense than most cultured pearls but differing sizes of the cultured quotseedsquot will produce a variety of densities O The majority are strung as necklaces Some are used in rings I Rejects are used in medicines as a source of calcium Care and enhancements of pearls O Conchiolin is prone to drying I If this occurs the pearl becomes dull the surface cracks and finally peels O Pearls are damaged by excess humidity dryness acids perspiration cosmetics hair sprays and other chemicals I Pearls are a totally impractical gemstone they39re soft vulnerable to humidityaridity crushingscratching and so on 0 Yet because of history marketing and inherent beauty pearls are still popular Since they are slightly porous pearls can be dyed to change their shade I Dyed black pearls can be detected by brushing with an acetonesoaked cotton swab the dye will come off on the swab Also dyed pearls are inert under LWUV while natural black pearls often uoresce a weak orangered color I Pearls are sometimes bleached to lighten them I They are soft and so can polish easily but this polish also dulls quickly if not cared for O Oils and wax have been employed to improve their appearance but excessive treatment will reduce their value Conch Pearls 0 Because they lack nacre these are not considered true pearls I They are often orange or pink in color I They form as concretions in conch shells Simulants 0 Most imitation or fake pearls are made of plastic or glass I They are often easy to tell from real pearls since the overlapping layers of nacre on a real pearl will quotsqueakquot when rubbed on one39s teeth Fake pearls are created by covering spheres or other shape with an enamel or laquerlike coating I This laquer will contain small re ective quot akesquot in suspension that give a pearlescent appearance Essence d39orient is made of a suspension of guanine an amorphous white material that is part of the membranous coating on fish scales and is then applied to the bead I Guanine is used in cosmetics paints and other places where a quotglitteryquot appearance is desired although modern synthetic materials and natural mica are often used instead Modern plastics can also be made to look very much like pearls I Majorca Pearls are a unique simulant pearl that are produced only off the coast of Spain on the isle of Majorca The glass beads are covered in the essence d39orient multiple times while polished and smoothed between coats I This produces a very lustrous appearance As far as simulants go Majorca Pearls are considered the most valuable 11 Amber 0 Amber is fossilized tree resin 0 Similar in comp to pine sap but it has undergone oxidation loss of volatiles burial 0 Physical Properties I Amorphous not crystalline I Conchoidal Fracture I Hardness 225 I SG 1011 will oat in very salty water SGll3 I Melts between 295 and 395 degrees Centigrade mixture I RI 154 I Chemical formula C10H16O I Luster resinous vitreous I Color Yellow to brown sometimes white red green or blue 0 The greek word for amber electron is the basis for the modern word electricity 0 When rubbed amber will produce a static charge attracting bits of dust lint and other small particles 0 Fake Amber 0 Plastics have been widely used to simulate amber I However true amber will oat in salt water while most plastics will sink I Also a heated needle when pressed into amber will produce a resinincense smell 0 Plastic fakes will smell like burning plastic I If you rub amber with your thumb it will smell like resin I Also amber is fairly resistant to organic solvents like acetone and toluene while most fakes are soluble in them 0 Copal is also fossil resin but it has not undergone as much of a chemical change as amber it will react to some organic solvents and uoresces differently under shortwave ultraviolet light I Most famously amber often contains trapped insects more rarely lizards and amphibians 0 The presence of inclusions is no guarantee of authenticity however I The ability to place insects and more wild things like scorpions and frogs into fake amber has been used to quotenhancequot these fakes 111 Coral I Coral is Calcium Carbonate aragonite and or calcite precipitated from seawater by tiny colonial organisms 0 Precious coral Corallium rubrum ranges from white to light pink to red 0 Tiny individual coral organisms called polyps precipitate a small exoskeleton of CaCO3 as a group these colonies can become quite large IV VI 0 Gilson imitation coral consists of calcite silica and small amounts of iron oxide as a chromophore I It also lacks the structural features of natural coral Since most coral growth is limited to warm shallow seas nearly all coral growth is restricted to the tropics about 30 degrees latitude north and south Ivory True ivory is dentin the material our teeth are made of 0 Only teeth that are quite large contain sufficient amounts of dentin to work into carvings Elephant and walrus tusks even the teeth of the giant sperm whale have been used for ivory Since the major sources of ivory are gigantic terrestrial mammals the ivory trade once an incredibly large industry has become both politically and ecologically disastrous 0 Hunting of elephants and other large mammals has decreased markedly in the last 50 years as populations of these animals have become perilously small Most quotecologicallyquot harvested ivory comes from extinct mammoths and mastodons recovered in the siberian tundra this material is sometimes exquisitely preserved trapped in the permafrost Ivory consists both of organic and inorganic material 0 The inorganic portion being crystals of calcium phosphate same mineral that39s in bone The crystalline portion is held together with collagen and elastin similar to the material in cartilage 0 Modern plastics are very similar to ivory in texture and color 0 Tagua nuts are also used to simulate ivory Jet Jet is similar to coal in that it is altered plant material 0 Primarily carbon with some hydrogen and oxygen it was once used in carvings It was popular for many centuries because it was easily carved and polished well producing a deep brown or black resinous appearance 0 quotJet Blackquot comes from this material Jet has a petroleumlike odor similar to coal 0 It is not used much today because glass and plastic are more durable and stable I In the book Les Miserables the main character Jean Valjean owned a factory where they made a synthetic material that imitated jet Shell Tortoise Shell Some mollusks shells have a desirable motherofpearl appearance on their own 0 Usually it is used for inlay work and in carvings I The decoration on the bows of stringed instruments usually have a mother ofpearl inlay 0 They are less often used as imitation pearls 0 Tortoise shell has a mottled appearance 0 But contrary to its name quottortoise shellquot comes from the hawksbill turtle a marine animal 0 Tortoise shell is mostly keratin the same material in fingernails animal horns and claws I The hawksbill turtle is listed as an endangered species and it is currently illegal to import this material into the United States VII Tagua Nuts 0 Tagua nuts are harvested from tropical trees 0 They are used as an alternative to ivory
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