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54 Cards in this Set
- Front
- Back
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visible range/roy g biv
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350 - 750
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electromagnetic spectrum is:
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gamma (short and high frequency) -low intensity radio waves (long wavelength and low frequency)
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spectral emittance curves and effects of illumnation type on colour
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describe the intensity of light at a particular wavelength and will often display data only across the visible spectrum
natural daylight and incandescent lamps: warm red fluorescent lamps: blue and green |
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alexandrite
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Variety of mineral chrysoberyl, Pleochroisn: colour change characteristics include garnet, corundum, and zultanite
Blue: Natural Light/Florescent Red: Incandescent |
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Reflection Law
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the angle of incidence is equal to the angle of reflection (<i = <r).
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total internal reflection
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the angle of incidence is greater than the critical angle
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isotropic mineral
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belong to isometric crystal system, have only one refractive index applicable in all 3 orientations, diamond, spinel
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anisotropic mineral
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Material from all crystal systems other than isometric show more than one refractive index
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tetragonal and hexagonal crystal systems have
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have two distinct refractive indices
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monoclinic, triclinicl and orthorhombic crystal systems have
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have three distinct refractive indices
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birefringence
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1 refractive index - 2 refractive index, low = blurry, high = doubled (2 paths)
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Dispersion
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the difference in the refractive index for light of the shortest and the longest wavelengths
emstones with higher values of dispersion will show greater spreading, or dispersion, of colour. Gives the gem Fire |
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Idiochromatic Coloration
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Self-coloured
ex. Peridot (Fe2SiO4), Turquoise (CuAl6(PO4)4(OH)8·4H2O) |
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Allochromatic Colouration
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require "impurities" to generate their colour
ex. emerald (Be3Al2Si6O18) with CR |
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Pseudochromatic minerals
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how colours and optical effects through dispersion and scattering of light. Colour and optical effects generated from scattering includes asterism, chatoyancy, iridescence, opalescence, and labradorescence.
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pleochroism
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display different colours (or saturation of colours) depending on the crystallographic direction of the stone being viewed nd is caused by differential absorption of light according to orientation of the crystal, Tanzanite, Alexandrite
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Transparency Five Main Groups:
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transparent, semi-transparent, translucent, semi-translucent, and opaque
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Beryl
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(Be3Al2Si6O18) that commonly forms hexagonal prisms
Hardness: 7.5 to 8, colourless, opaque, Pegmatite -oiling to fill cracks with same refraction index type III most valuable Can be produced synthetically |
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What Colours can Beryl Have?
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-Beryl Emerald green Cr+3, V+3 for Al+3, Metasomatic zones
-BerylAquamarine, light to dark blue-green,Fe+2, Fe+3 for Al+3, and often Na+ in the channel, Pegmatite (most common, least valuable) |
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three main types of secondary deposits for Beryl
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elluvial, colluvial, alluvial
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Why is Beryl rare?
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there is very little of the element beryllium in the upper continental crust and it concentrates only in specific rock types, such as granites and pegmatites.
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Emerald's premier locality
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Columbia
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Corundum
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Al2O3
aluminum oxide that commonly forms hexagonal barrel-shaped prisms that taper at both ends or as thin tabular hexagonal plates Hardness: 9 No cleavage Has mineral and fluid inclusions, low dispersion, high density, and two refractive indices -heating to fix -can be produced synthetically |
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Ruby
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Corundum: The red region of the electromagnetic spectrum (~650 nm) does not have very much absorption at all and results in all colours but red being blocked by ruby.
When Cr is introduced into corundum it makes the mineral fluorescent under UV light. Best rubies have little Iron |
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Blue sapphires
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Corundum: Fe+2 and Ti+4 substituting into the crystal structure for Al+3. The process of intervalence charge transfer (essentially continual swapping of electrons, bouncing back and forth) occurs between the Fe and Ti and all colours except blue are absorbed.
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the four C's
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colour, clarity, cut, carat, (plus country for corundums)
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Metamorphic Corundum Deposits
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provide a significant volume of gem corundum from primary sources. More importantly, it is this primary deposit type that produces the most significant quality of rubies and sapphires.
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xenocrysts Corundum Deposits
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Great depths below the continents, in the upper mantle regions that are favourable for sapphire growth are "tapped" by magmas rising towards the surface. When these crystals become entrained (or caught up) in another magma, such as an alkali basalt or lamprophyre.
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Are corundum's rare?
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No, but untreated peigon red rubies and cornflower blue sapphires are!
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Basic Tools of the Trade (8)
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Unaided eye, tweezers, rock hammers, chopsticks, scratch pads/streak plate, hardness pick.
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Gemological Tools
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Loupe, Chelsea Filter (sees colours), Dichroscope/Polariscopes(determining what optic class, 2/3 refraction indices), UV lamps (used to observe UV fluorescence under short wave and longwave), Refractometer, spectroscope,
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inclusions
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the tiny gases, liquids, and solids that exist in every gemstone
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Mineralizing tool
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Polarizing Microscope
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Pegmatites supply the world with the best
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tourmaline, topaz, and beryl
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What is pegmatite?
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Intrusive igneous rocks that are texturally very coarse to gigantic in size, filled with many gem minerals and rare elements
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Tourmaline
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Pegmatite gem: XY3Z6(BO3)3Si6O18(OH)4, schorl: NaFe3Al6(BO3)3Si6O18(OH)4
Complex borosilicate mineral group with hexagonal symmetry( long slender crystals with a pseudo-hexagonal outline and euhedral crystals),two poor cleavages, fairly dense (SG ~ 3.2), hardness of 7, commonly opaque black -heated and epoxy treatment -can be synthetically made -semi-precious and hard to value because of diversity |
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Watermelon tourmaline
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Bi-colour variety of this mineral where a bright pink core (from Mn) is surrounded by a grass green rim (usually from Fe). This colour gradient is the result of changing geochemical growth conditions where originally the system was Fe-deficient, leading to the Mn-dominated pink colouration.
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Topaz
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Pegmatite Gem:
Al2SiO4(F, OH)2 orthorhombic crystal system and usually forms prismatic crystals with an eight sided cross-section aluminosilicate mineral containing fluorine (F); often, appreciable hydroxyl groups (OH)- replace F. perfect basal cleavage, hardness of 8, placing it above quartz and tourmaline but below beryl and corundum, fairly dense with a SG of ~3.5, colourless when pure, ightly coloured brown, blue, and yellow are the most common colours; pink, red, and lavender round out the mix. Imperial topaz is valuable (Brazil) -Irradiation, heating, and coating to treat -Can be synthetic |
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Spodumene
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Pegmatite:
LiAlSi2O6 Lithium (Li)-bearing aluminosilicate, base mineral for the pink kunzite (common) and green hiddenite. Originally colourless to light pink. Pyroxene group minerals spodumene forms prismatic crystals with square or rectangular outlines,two distinct cleavages that run parallel, hardness of 6.5, moderate specific gravity of ~3.2.,lengths with some up to 12.5 m. -Heat to treat -Syntheticable - Imitations for emeralds and morganite -pretty cheap |
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Pegmatite Pocket
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Where gems tend to be found in pegmatite rock
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Pegmatite Genesis
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From parental pluton, granite, magmatic history - granite undergoes fractionization (sequential crystallization of minerals as granitic magma cools), and residual elements are leftover, highly fractionated pegmatites are often called fertile
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Four Pegmatite Groups
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Abyssal (high temperature, variable pressure);
Muscovite (low T, high P); Rare-element (low T, low P); and Miarolitic (medium T, low P) |
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three main factors for Pegmatite Group organization
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1.depth of emplacement below the surface;
2.range of temperature; and 3.type of rare element enrichment |
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Pegmatite Zone
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Outer - Inner, Border (tourmaline, garnet, feldspar & quarts), Wall (feldspar, quarts, garnet, beryl), Intermediate Core Margin (coarse crystals, gemstones begin to appear, feldspar, tourmaline starts to change), Core (pockets, gem quality crystals, beryl speudomine, kunzite)
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Why are gem bearing pegmatites rare?
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- environment needs abundant granitic rocks
- magmas need to evolve and fractionate - fertile -wide enough dykes that encourage pocket growth - high volatile concentration - tectonic environment - steady erosion |
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Most famous pegmatite locale
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Minas Gerais, Brazil
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cabochon, name three types
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do not readily form transparent crystals appropriate for faceting
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Lapis lazuli
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lazurite, sodalite
afghanistan |
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Jade
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Jadeite is a rock (i.e., polymineralic) comprised primarily of the pyroxene mineral jadeite (NaAlSi2O6)
Nephrite is actually a mass consisting primarily of finely crystallized (microcrystalline to cryptocrystalline) amphibole with a composition between tremolite to actinolite (Ca2(Mg,Fe)5Si8O22(OH)2) |
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Turquoise
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Cabochon: Some of the more famous deposits are in Egypt, Iran, and the United States.
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Quartz
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-12 gem variations, most precious = opal, amethyst, citrine, and agate.
-hardness 7 -global distribution |
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Garnet Group
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Ugrandite and pyrelspite are two subgroups
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Pearl (organic gems)
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most pearls are cultured
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amber
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fossilized coniferous tree resin, preserves animals
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