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What is a Gemstone 01 08 2014 Gem Material that is solid and sufficiently attractive and durable to be used for personal adornment usually after cutting and polishing Usually a gem is o Naturally occurring although it may be synthesized in a lab o Desirable for its beauty durability rarity fashion and portability What is a precious stone Precious stones and Semi precious stones are used to distinguish more and less valuable stones These terms have been arbitrarily applied for marketing No uniform criteria or definition in the differences between precious stones and gem stones and semi precious stones All will be considered gemstones if they are beautiful and solid Beauty Key attribute of a gemstone Beauty is in the eye of the beholder May be color brilliance sparkle fire rainbow effect transparency or a combination of all of these Brilliance Why do faceted gems sparkle o Facet flat polished surface o Table Top facet often largest o Crown bezel top top of the stone o Girdle Edge between crown and pavilion o Pavilion lower part of the stone o Culet small facet at the bottom o Brilliant cut is a round cut Why are gems faceted o To take advantage of the behavior of light that passes into the gem Depends on the reflection and refraction of light Reflecting the bouncing of light off of a surface facet Refraction the bending of the light as it passes from one substance air into another gem Facets cut below the girdle pavilion are cut to bounce light and eventually come out of the table and allow for the stone to appear to light up o Reflection bouncing of light off of a facet Internal reflection of light within the pavilion is needed to achieve the brilliance and sparkle Critical angle the angle at which total light reflection is achieved If light inside a gem is Inside the critical angle it exits the gem short pavilion Outside the critical angle its is internally reflected and then exits too long of a pavilion a European cut o Refraction Refractive Index RI the degree to which light is bent as it passes from air into a gem Determined by the density difference between air and a gem o Placement of each facet is determined using the critical angle o Critical angles are determined from the RI of the gem Fire o Rainbow like flashes of color in gems o Calcite makes up lime stone and looks like a diamond in relation to the amount of fire o What causes fire Dispersion or prism effect is the separation of white light of its rainbow colors The amount of light that is bent refracted depends on the wavelength color of the light Violet light bends more than red light Measured as the RI of violet light the RI of red light Not all gems are faceted o Some are just polished o Ex Pearls o Cabochon Gem cut with a rounded upper surface Luster o Appearance of light as it reflects off the surface of a gem o If the surface is rough light is lost o But when the surface is polished light is reflected o Various names are applied vitreous waxy pearly metallic Durability o Hardness resistance to scratching Mohs Scale of Hardness the scale is relative Harder Greater Value Diamonds Corundum rubies and sapphires are especially desirable 1 Talc Baby Powder Cosmetics 2 Gypsum Dry Wall 3 Calcite Limestone Tums and Antacids Fingernail can scratch 3 Aragonite and Calcite are used as pearls 4 Fluorite Fluorescent Toothpaste Water 5 Apatite Bones and Teeth 6 Orthoclase Feldspar 7 Quartz Most common mineral at the earth s surface Sand in deserts and beaches glass the most gemstone varieties flint amethyst citrine abundance in the atmosphere as dust 8 Topaz 9 Corundum Red ruby all others are sapphires 10 Diamond o Toughness resistance to fracturing and chipping Jade is desirable even though it is softer than diamond because jade is much less brittle o Stability resistance to color loss due to heat light or chemicals The purple color of amethyst is not a very stable color Value The four Cs o Cut o Color o Clarity Ideal proportions of a gem to optimize brilliance and fire to and result in greater value Some colors are more valuable then others The relative freedom from flaws inclusions and cracks o Carat Weight more valuable If cut color clarity are all held equal the heavier the How to identify a Stone 01 08 2014 Factors to Identify Stones Color is not the most important o All blue stones are not sapphires should be the last thing considered Transparency o Transparent o Translucent o Opaque Phenomena o Inclusions of other minerals Aventuresnecne Reflection of pieces inside o Star Affect Asterisms o Change color in the light Pleochroism multiple colors under different lights Alexandrite Type of light Angle of light Direction of the light Ultraviolet light fluorescence vs torch light appearance Luster o Metallic o Glassy or Vitreous o Earthy looks like dirt no luster o Adamantine Diamonds o Pearly o Waxy o Greasy o Resinous o Cubic Zirconia is almost 4x more than diamond Fire Dispersion Heft o Relative Weight Cubic Zirconia is relatively heavy So is hematite which is iron ore Where do Gemstones Come from 01 08 2014 Where gemstones form Igneous Environments o From molten rock At the earth surface blown out of a volcano A few meters down in the earth Diamonds form deep in the earth Sedimentary Environments o In rock formed by deposition of grains or precipitation of o Sediment is deposited rock that has been transported by wind minerals in water and water o Most important for finding gem quality diamonds Organisms Metamorphic Rock o Shells or bones of animals tree sap etc o In rock changed by heat and pressure Igneous Environments Some gems crystallize from magma o Either in molten igneous rock or in bubbles of dissolved gasses Pegmatite o Magma chambers are typically 20 50 miles across Ex Zircon topaz beryl o Unusual igneous bodies containing large crystals As magma cools water originally present in low concentrations becomes concentrated in the remaining melt because it does not get incorporated in the early formed materials Also some rare elements like beryllium Be and boron B are concentrated in the melt When the last of the melt finally crystallizes there is enough water left to Hydrothermal Deposits o Hydrothermal deposits involve water derived from the cooling of magma bodies or heated groundwater o Gems or metals crystallize from solution in cracks or other open spaces in rock Veins of minerals fill cracks Ex Tourmaline quartz gold beryl Gems formed in the mantle o Olivine peridot is one of the most abundant minerals in

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OSU EARTHSC 1108H - Gemstone

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