What are Gems Gem sufficiently attractive and durable for personal adornment o Desirable because of beauty durability rarity fashion and portability Precious Stone everything except diamonds Beauty key attribute of a gemstone o Color brilliance sparkle transparency fire luster o Sparkle because of facets flat polished surface Facets o Table top o Crown top of stone o Girdle edge btw crown and pavilion o Pavilion lower part of stone o Culet small facet on bottom instead of point Reflection bouncing of light off a facet o Internal reflection within the pavilion is needed to get brilliance o Critical angle angle of internal reflection Refraction degree of bending of light as it passes from air into gem o Critical angles are determined from RI of gem Fire rainbow like flashes of color o Dispersion separation of white light into a rainbow of colors o Violet light bends refracts more than red light Luster appearance of light reflecting off the surface of a gem Durability o Hardness Resistance to scratching Moh s Scale of Hardness 1 Talc make up 2 Gypsum plaster 3 Calcite calcium 4 Fluorite toothpaste 5 Apatite Ivory 6 Orthoclase Feldspar 7 Quartz dust 8 Topaz 9 Corundum 10 Diamond o Toughness resistance to chipping or fracturing Ex Jade is tougher than diamond o Stability resistance to color loss due to heat light chemicals Ex Amethyst 4 C s 50 when cut o Cut ideal proportions cut to maximize brilliance and fire lose about 40 Brilliant emerald oval marquise pear cushion asscher radian princess trillion heart o Color some colors are more desirable More yellow less valuable fancy stones are colored most expensive is blue diamond o Clarity inclusions minerals liquids or gases contained within a gem cracks decrease value Except for amber o Carat Weight larger will be more valuable 1 carat 2 grams 1 carat 100 points 2 carat stone not worth 2 times a 1 carat stone but more like 2 25 times more Valuation o Rarity o Fashion o Portability Natural and Laboratory Practices Igneous environments o Crystalize from magma o Pegmatite unusual igneous bodies containing large crystals Inside earth magma chambers cool Beryllium rich pegmatite Beryl chyrsoberyl o Hydrothermal deposits gems or metals crystallize from solution of water in cracks of cooling magma o Gems formed in mantle Olivine most abundant diamond o MAFIC dark and ultra dark Sedimentary Environments o Alluvial deposits rock is weathered at the earth s surface crystals are released as grains Washed into streams Deposits of metals are placer deposits Diamonds rubies sapphires beryl weather from igneous or metamorphic rocks then become concentrated in alluvial deposits o Precipitation of gems if physical conditions of the water containing dissolved ions changes minerals precipitate from solution Minerals that form depend on what dissolved elements are present Metamorphic Environments o Plate tectonics creates metamorphic enviornments having high temperatures and pressures Metamorphic zones large volumes of rock are changed in o Processes flame fusion hydrothermal flux growth ceramic techniques response to heat and pressure Ex Jade garnet Synthetic gems created in labs skull melting Gems found o Mining diamonds rubies sapphire o Alluvial Deposits diamonds sapphires emeralds o Collecting from sea pearls coral shell o Sedimentary Deposits amber ammolite Crystals Crystal solid with faces Amorphous Gems non crystalline solid with no faces flows Silicates Si rich minerals Important Minerals of gems silicon aluminum oxygen carbon o Ex Quartz veryl tourmaline garnet chips in computers o 75 of earths gems Oxides O rich minerals o Ex Corundum alexandrite o Negatively charged ion o Most expensive gems in this category Carbonates C and O o Ex Calcite aragonite Elements o Diamond C gold Au Silver Ag Platinum Pl Atomic Bonding ions form when atoms gain or lose electrons o Anion Negatively charged ion o Cation Positively charged ion Ionic Bond electrons are exchanged o Weak bond no gems made at ionic bond Covalent Bond electrons are shared o Strongest bond Van der Waals Bond weakest bond present on cleavage planes of minerals o Sheets remain intact but bonds are broken when pressure ex Graphite Metallic bonds electrons are free to travel through the substance o Gives metals their cohesiveness malleability ductility heat conductivity and electrical conductivity How are crystals formed o Arranging atoms in a regular pattern Unit cell basic arrangement of atoms in a crystal structure Polymorphs solids having same chemical composition but different crystal structures ex Diamond and graphite Symmetry faces on a crystal have a symmetry because of arrangement of atoms 6 Crystal Structures o 1 Cubic SR no preferred direction for light to pass through from point to o 2 Hexagonal DR fastest light propagation is down the long axis most center is the same Ex Diamond garnet amber popular in gems Ex Beryl corundum o 3 6 Tetragonal orthorhombic monoclinic triclinic What gives minerals their color o Idiochromatic gems color causing elements are an essential part of the chemical composition of stone self coloring stones Ex Rhodolite garnet o Allochromatic gems gems are colorless when pure color produced by impurities Ex Corundum Physical Observations o Cleavage preferential breakage along some surfaces where atomic bonds o Fracture breakage along a surface that is not related to its internal atomic are weak structure o Streak color of powdered mineral Crystal Shapes o Pyramidal acicular needle like massive rock dendrite branch like amorphous no shape twin internal structure repeated Pleiochroism color change in different light sources ex Alexandrite Isotropic crystal always remains dark under cross polarized filters SR Anisotropic light passes through the crystal as it is rotated under the cross polarizers DR Diamonds Origin from French people used to smash diamonds with hammers as a test Composition Carbon Hardness 10 SG 3 52 SR Cubic System covalent bond Formation Luster Adamantine o Form near core mantle boundary under high temperature and pressure conditions o Carried to surface as inclusions in kimberlite pipes must rise rapidly Graphite is stable form of carbon diamond will burn if heated to bright red 1725 diamonds discovered in alluvial deposits in Brazil 1859 diamonds discovered in south African deposits 1870 diamonds discovered in kimberlite pipes 1871 diamonds discovered on De Beers farm Big Hole Diamonds mined in South Africa Australia Brazil Russia De Beers o