Gemology 306
COURSE SUMMARY

  1. What is a gem ? - Necessary properties:
  2. Appearance:

    * Luster - reflected light

    - Cutting proportions to ensure that light returns out of the top of the stone and does not 'leak' out below

    3. Value:

    NOTE: Dispersion, critical angle, and refractive index are diagnostic characteristics !

    4. Where are gems found / formed ?

    1. Alluvial deposits : resist weathering; survive transport in streams etc., -accumulate in depressions in river beds etc. e.g., gold, platinum, rubies and sapphires
    2. Near-surface chemical deposits - leaching of certain elements by percolating rain water and precipitation in other rock units

      e.g., malachite, opal

      3. Hydrothermal deposits (hot water): late-stage crystallization of melts rich in unusual elements (boron, beryllium, etc) - coarse-grained granite-like rocks).

      4. Magmatic: (molten rock): peridot (olivine), rubies, sapphires, topaz: inclusions in magma or precipitated in gas cavities of lavas.

      5. Metamorphic deposits: (rocks changed by heat and pressure): e.g, garnet; jadeite (e.g,. formed in subduction zones (where crust under oceans plunges down into mantle at continent margins etc.)).

      6. Deep mantle - high-pressure and -temperatures: kimberlite pipes extract diamond-bearing rocks and transport them to the surface extremely fast (from depths of over 90 miles below the surface).

      5. What is a crystal:

      1. Crystals are regularly repeating three dimensional arrays of atoms that are held together by chemical bonds (ionic (+: -) or covalent (electron sharing).

        Anions (-ve charge) arrange themselves around cations (+ve charge).

        Number of anions around a cation is determined by the relative sizes of the anions and cations. The arrangements are described as coordination polyhedra.

        Crystals are grouped into one of 6 crystal systems based on the symmetry of their atomic arrangements :

        • cubic (isometric)
        • tetragonal
        • hexagonal
        • orthorhombic
        • monoclinic
        • triclinic.

        Symmetry of the arrangement of atoms within crystals is reflected by the regular geometric form of many (uncut) crystals.

        Minerals are grouped according to their composition. Silicates are subdivided on the basis of the way in which their tetrahedra are linked.

        6. How are gems identified ?

        1. Optical properties to work out which of the 6 crystal systems the stone belongs to (often eliminate most look-alike alternatives).

          The gemological microscope: using reflected or transmitted light (dark field or direct illumination).

          • fitted with two pieces of polaroid (oriented at 90 deg. to one another), one above and one below the stone: this allows evaluation of whether the stone is
          • isotropic vs. anisotropic -pleochroic.

          Light optical properties using plane polarized light:

          A: (a) Isotropic ( viewed between two pieces of polaroid the stone remains dark regardless of orientation when rotated): thus the stone is cubic (isometric). Has only one RI.

          (b) Anisotropic: Uniaxial: stone remains dark under crossed polars when rotated if, and only if, it is viewed along its optic axis (this is the unique axis of the crystal). Other orientations light is transmitted. Stone has 2 RI's. Stone must be assigned to the tetragonal or hexagonal crystal system. Termed 'double refracting', like in calcite.

          (c) Anisotropic: Biaxial: Stone remains dark under crossed polars when rotated if it is viewed along one of two special directions (i.e., it has two optic axes). Stone has 3 RI's; stone must be assigned to the orthorhombic, monoclinic, or triclinic crystal system.

          B: pleochroism: in polarized light (bottom polaroid sheet only) the stone changes color when it is rotated: due to absorption of different wavelengths of light along different directions.

          C. Consequences of anisotropism: cut uniaxial stone with optic axis perpendicular to the table: see single facet image; cut with optic axis parallel to table: see doubled facets. The doubling of facets indicates a stone is aniostropic (thus not cubic=isometric).

          2. Specific gravity -weight of one cubic centimeter of mineral divided by the weight of one cubic centimeter of water (i.e., density / 1 g/cubic centimeter).

          SG thus has no units (it is just a number). SG's range from ~ 1 to 7 and are often very diagnostic.

          SG's are measured by : (weight of mineral in air)/ (weight in air - weight in water).

          Also, SG's can be measured by matching the SG of the stone with that of a special fluid with known SG. When the fluid's SG matches that of the stone the stone will neither sink or float.

          Note that the specific gravity is determined by what elements are present and how they are closely (or densely) they are packed.

          Compare the specific gravity of two forms (polymorphs) of carbon (diamond = 3.52, graphite = 2.23).

          3. Refractive index:

          • refractometer to measure the critical angle to determine RI
          • or by immersion method: using liquid of known RI: when RI liquid matches that of the stone the stone is almost invisible
          • or by focusing method in optical microscope (RI=thickness divided by apparent thickness: see text).

          More on gemstone identification:

          1. thermal properties are a useful diagnostic tool (note that some substances feel cold (e.g., glass) and some (often plastics) do not - can be measured quantitatively. They vary enough to be quite diagnostic.
          2. Spectroscopies: based on passing radiation (e.g., light) through a crystal and examining the distribution of wavelenghs in the light that comes out.

            -These techniques are especially useful for detecting water bonding environments (and OH-groups), organic compounds (such as impregnating glues and fillers), and demonstrating heat treatment.

            • Mineral compositions: Electron microprobe techniques
            • Color in minerals:

            1. how do we perceive color ?

            • type of illumination: incandescent vs fluorescent -eye: rods and cones
              -stone: most often absorption phenomena

            3. Color due to:

            (1) dispersed metal ions
            (2) charge-transfer phenomena
            (3) color centers
            (4) band theory - diamond color - not discussed in detail here (5) physical phenomena (e.g., diffraction): discussed later

            4. Individual dispersed metal ion: affect dependent on

            • valence state -neighboring atoms
              -ion coordination
              -orbital orientation
              -details of coordination polyhedra
            • charge-transfer: from oxygen-cation or cation-cation (intervalence): transfer absorbs energy:
            • Color centers: radiation -> trapped electrons that absorb energy (released by heat): e.g. spodumene, topaz, halite, fluorite, smoky quartz, beryl

            GEMS

            Diamond:

            1. Diamonds formed in the early half of the earth's history, most are dated at 990 million years - some are over 3,200,000,000 (3,200 my) years old.
              • Dates are determined using uranium-lead (U-Pb) isotopic techniques - small inclusions are dated, not the diamonds themselves.
              • Diamonds are formed deep in the earth and are transported to the surface in kimberlite pipes - the transport must occur rapidly to prevent diamond -> graphite !
              • The molten rock that travels up the kimberlite pipes is commonly not nearly as ancient as the diamonds - the diamonds are inclusions broken from underlying rock units and simply carried in the molten rock.

              -Diamonds are commonly mined from alluvial deposits.

              Basic statistics: Hardness = 10
              Dispersion = 0.044
              Crystal system = cubic
              Cleavage - 4 cleavage planes - rounded diamonds in alluvial gravels do to impact chipping

              ~ 20 % of diamonds mined are suitable for gems

              Factors affecting value : Color, Clarity, Cut, Carat Weight

              (visual or instrumental color grading: blue-white to yellow / clarity on a flawless to imperfect scale / cut: very good to poor)

              Value enhancement : crack filling, irradiation, heat treatment, drilling inclusions

              -irradiation to change color - indicated by concentration of color at culet or keel line of cut stone

              Diamond simulants: - recall the read through effect, the reflection pattern, other physical and optical properties

              CZ, GGG, synthetic spinel, YAG, strontium titanate, rutile etc: marked under multiple names!!

              Synthetic diamonds: high-pressure and high-temperature synthesis involving flux - inclusions of flux may be found in synthetic diamonds : synthetics are supposedly not released by manufacturers for gem purposes.

              Corundum:

              Ruby:

              Composition : Al2O3
              Color: red and shades of pink
              Crystal system : hexagonal
              Pleochroism : strong
              Absorption and fluorescence: red
              Color due to chromium impurity (Cr) ~ 4 % brown due to Fe

              Origin of Rubies: metamorphosed marbles, basaltic rocks, alluvial deposits

              Synthetics: large crystals and star ruby

              Sapphire:

              same composition, hardness, crystal system, pleochroism strength as ruby

              Color: due to Fe-Ti charge transfer, varying the proportions of Fe and Ti will change the color
              Inclusions: liquid and gas bubbles - heat treatment explodes these inclusions

              Ruby and Sapphire:

              • rutile needles: oriented-> asterism (star sapphire - and star ruby) (rutile = titanium dioxide) - changed by heat treatment
              • synthetics: wispy clouds, curved striations, visible seed crystals

              -d iffusion treatment (mostly for sapphire: addition of impurities to crystal edges - enhances color - result is uneven

              Various combinations of heat and diffusion and their results were reviewed

              Beryl

              Varieties include: Emerald, aquamarine, heliodor, morganite, goshenite Crystal system: hexagonal
              Hardness = 7.5 - 8.0
              Specific gravity = 2.63 - 2.91 (low compared with many gems) Color: Chromium (Cr)-> green of emerald Iron (Fe) -> greenish blue, golden yellow Manganese (Mn) -> red beryl and morganite

              Flaws: Most emeralds are flawed and contain inclusions - crystals are very brittle
              Many aquamarines, heliodors, morganites are flawless!

              Inclusions - may be 3-phase (solid + liquid + gas), e.g., Columbian emeralds

              Cuts: the "emerald cut" was developed because emeralds are sensitive to knocks and bangs.

              Aquamarine: Fe-Fe charge transfer, heating improves color if Ferric (3+)-> ferrous (2+) iron.

              Heliodor and golden beryl: Fe-O charge transfer

              Maxixe: fading blue !

              Goshenite: colorless

              Morganite: soft pink - violet: (sometimes peach or salmon) - color may be improved by heating

              Other treatment: oiling - filling fractures

              Topaz:
              Composition: aluminum silicate containing fluorine and hydroxyl Crystal system: orthorhombic
              Hardness: 8
              Cleavage: strong basal (planar) cleavage Specific gravity : 3.5
              Color- yellow-brown-orange-colorless-blue: note importance of color centers (trapped electrons) - removed by heat treatment. Synthesis: rare

              Geological origin: late stages of solidification of molten rocks - in cavities in lavas and granite, and in alluvial deposits.

              Treatments: Irradiation +/- heat treatment: difficult or impossible to detect alpha, beta, gamma particles/ rays
              Facilities used: gamma-ray facilities: g-rays

              linear accelerators- high energy electrons

              nuclear reactors: high energy neutrons

              various colors based on combinations of treatments. green tint due to yellow color centers removed by heat treatment - desirable result of treatment is blue without a steel-grey or green tint

              Zircon:

              Composition: zirconium (Zr) silicate
              Hardness: 6.5-7.5
              Specific gravity : 4.6 - 4.7 (this is high) Crystal system: tetragonal thus optics- uniaxial RI high, dispersion high, cut stones exhibit brilliance and fire. Fracture: conchoidal
              Color: green-yellow-red-brown-blue

              Geological: zircon is an extremely common minor mineral in many rocks; resistant to weathering, often found in alluvial deposits.

              Treatments: to change color
              Brownish -red @ 900 C in air-> colorless or golden in reducing environment (no air) -> colorless-blue

              • most colorless and blue zircons have been heat treated!
              • UV or sunlight can modify color in some cases!

              Distinguished from diamond by greater tendency to wear and by double refraction (matara diamond = zircon)

              Important characteristics: contain U and Th -radioactive decay used to give age dates for minerals and rocks

              • radioactive decay -> structural damage ("metamict") - appear cloudy (high-intermediate-low (metamict) refer to increasing sequence of damage)

              Radioactivity in minerals:

              Detection: geiger counter etc.
              Many ways of measuring radiation - esp. with regard to reporting potential danger to humans (radiation given off, exposure, dose absorbed, damage done, etc.)

              Dose should be considered as a function of time when comparing gems - different half lives for different isotopes- dose for zircon (natural) over 10 years is comparable to that from an irradiated (blue) topaz.

              Chrysoberyl - cats eye crysoberyl and alexandrite variety

              Three varieties - differ in impurity and inclusion content Hardness = 8.5 - one of the harder gems Composition: a beryllium-aluminum oxide containing Fe and Cr impurities Fe -> yellow color
              Crystal system: orthorhombic thus biaxial Color: yellow, green, brown

              Geological origin: Be-rich pegmatites, metamorphosed limestones.

              Alexandrite effect: due to absorbtion of energy by Cr (3+) - wavelengths transmitted are blue-green and red - Natural light is rich in blue, candle light is rich in red.

              Thus, if put in mostly red light (no blue to transmit) looks red. If put in mostly blue, looks blue.

              Other gems exhibit the Alexandrite effect (garnet, sapphire, etc.)

              Chrysoberyl- variety 'cat's eye': cabochon cut from stones with needle-like inclusions or cavities - cut so that needles parallel the base and are perpendicular to the long dimension of the oval shape.

              All varieties of chrysoberyl can be synthesized.

              Tanzanite- variety of zoisite, deep blue, H = 6.5-7, a silicate mineral - heat treated to improve blue color

              Spinel: metal oxide - metals include Mg, Fe, Al, Cr etc. Cubic mineral,. H= 8, SG=3.6 occurs as octahedral crystals. Common minerals. Red spinel often confused with ruby.

              Rock-forming minerals:

              Quartz: SiO2 - a framework silicate (all tetrahedra share corners). H= 7., SG=2.6, Hexagonal - crystals often occur as hexagonal prisms. Clear quartz: technologically useful.

              • Amethyst: purplish quartz - color due to Fe (involves charge transfer and irradiation).
                -Occurs in geodes in lavas (often associated with agate). Twinning distinguishes natural from synthetic.

              -Citrine: yellow-brown quartz - produced by heat treatment of amethyst.

              -Smoky quartz: Aluminum-bearing, irradiation induced color (natural or man-made) - heat treatment restores clarity.

              -Rutilated quartz: rutile needles

              -Rose quartz (pink color); milky quartz; adventurine (inclusions of mica), tiger's eye (replacement of asbestos).

              Feldspar: Na-Ca-K silicate minerals - major constituent of crustal rocks!

              • feldspars are monoclinic or triclinic minerals, H=6-6.5, SG= 2.6-2.8.
              • Divided into two groups: alkali (Na-K) feldspars and plagioclase (Na-Ca feldspars).
              • moonstone and plagioclase show rainbow colors: adularescence or schiller due to intergrowths of two feldspars.

              Olivine: gem form is peridot: major constituent of mantle rocks. Yellow-green mineral,
              H = 6.5-7, SG=3.22-3.4,
              orthorhombic,
              Mg-Fe-silicate - island silicate because silicon tetrahedra are not connected.

              Also found in meteorites, basalts, etc.

              Color depends on Fe content - sometimes crystals are zoned (change color across xal).

              Iolite = cordierite
              blue,
              H=7-5.5
              orthorhombic gem,
              Mg-Al silicate mineral with a structure somewhat similar to beryl (ring silicate).

              Found in metamorphosed sediments.

              Pyroxenes:
              single chain silicate (tetrahedral pyramids containing silicon linked to form infinite chains).
              Two perfect ! cleavages at right angles. Destroyed by weathering thus not an alluvial gem.

              -spodumene: Lithium aluminum silicate (Li is the lightest metal on the periodic table): kunzite is the gem name form purple-pink spodumene, hiddenite for green (contains Chromium). Gems are strongly pleochroic, and difficult to cut because of good cleavage.

              • Jade = either jadeite (pyroxene) or nephrite (not a pyroxene-see below) - metamorphic rocks.

              -jadeite: sodium-aluminum silicate (chain silicate), monoclinic,
              small interlocking crystals.
              H= 6.5-7; SG = 3.34.
              Color due to presence of Chromium->green, Fe->paler green. Interlocking crystals in random orientations removes problems with excellent cleavage of individual crystals.

              Nephrite: an amphibole -
              a double chain silicate (two chains rather than one make up the backbone of the structure).
              Two good cleavages at 60 degrees.
              Strongly resembles jadeite.
              Made up of interlocking fibres.
              H= 6-6.5;
              SG = 2.95. SG distinguishes this from jadeite.

              Tourmaline:
              probably the most colorful gem - almost any color is possible. Crystals occur very typically as prisms with rounded triangular terminations. It is a complex (very complex) boron-bearing silicate. Interesting color variation in single xals - including 'watermellon tourmaline'.

              Displays pyroelectric and piezoelectric properties.

              Forms in pegmatites.

              Sometimes heat treated (may be destructive - makes gems brittle); also treated by irradiation.

              Garnet:
              cubic (dodecahedral crystals)
              H = 6.5-7.5;
              SG = 3.6 - 4.3
              Note very large range in both is due to very large range in possible composition. Many garnets are Mg,Fe,Ca,Al silicates.

              Two groups:
              UGRANDITE = uvarovite - grossular - andradite and PYRALSPITE = pyrope, almandine, spessartine

              -rhodalite = purplish garnet (variety of almandine) -malaia and color change garnets - mixtures of pyrope and spessartine -hessonite ~ essonite (cinnamon stone) = tsavorite (vanadium-bearing) -demantoid = variety of andradite - Cr-rich. Horsetail inclusions are characteristic
              -note YAG, GGG (diamond simulants) are garnets

              Form commonly in metamorphic rocks, may be very large crystals. Characteristic xal form (dodecahedra) has diamond-shaped faces - looks like a faceted ball.

              PRECIOUS STONES:

              Lapis Lazuli

              • blue gem material in use for > 6000 yrs. Mostly composed of the mineral lazurite, along with calcite (best if less), and pyrite (o.k. if not too much).
                A sulfur-bearing silicate mineral - a feldspathoid mineral (like Silicon-poor feldspar).

              -blue color due to charge transfer involving sulfur. H=5.5,
              SG = 2.7-2.9
              used as beads, carving. Sometimes dyed, coated with wax.

              Turquoise:
              ancient blue-green gem material- imitations date back to very earliest known examples !
              Cryptocrystalline material (tiny crystals) - triclinic, H = 5-6,
              SG = 2.8,
              a copper-phosphate mineral contains some Fe (makes it greener rather than blue).

              Often is impregnated with plastics, parafin, oil.

              Synthesized, simulants (esp. glass, plastic etc).

              Malachite - Azurite

              Copper-carbonate mineral;
              malachite is green, azurite is blue (azure blue). H = 3.4-4;
              SG = 3.75-3.95.
              monoclinic.

              Materials are aggregates - malachite typically shows pronounced color banding, nodular forms.

              Formed near the earth's surface by reaction of Cu-bearing solutions with carbonate rocks (i.e, in caves). Beads, carving.

              Opal:

              Consists of amorphous (hydrous) silicon spheres, color due to diffraction of specific wavelength of light. - play of color. H = 5.5-6.
              Precious opal (iridescent), fire opal (red/orange), common opal.

              Enhanced by impregnation with plastic; carbon particles at surface (from burnt paper or manure or sugar).

              Simulants - glass is common.

              Near-surface formation - wetting and drying in cracks - from silica-rich fluids. Esp. from Australia.

              Chalcedony / Agate:-
              fibrous,
              porous quartz ( porosity allows dying). Often as botryoidal masses.

              Color banding, tree-ring appearance - form by infilling cavities (last deposited band have smallest radius).

              Pearls:

              cultured and natural;
              made of calcium carbonate (calcite and aragonite) and conchiolin. Nacre - coating - must be aragonite.
              Form as concretions around irritant inside mussel / oyster (fresh and salt water).
              H = 2.4-4.5,
              SG = 2.7.
              Color: body color/ overtone (filmy surface color) / orient (iridescence). Concentric layers around irritant.
              Conchiolin is the binding agent, aragonite xals fill cracks in radiating array.

              Cultured vs natural - by structure - determined by XRD. Also conch and blister pearls.