Education & Editorials

  • Specific Gravity

    which-blue-stone Which is sapphire and which is tourmaline?

    Specific gravity (SG) is often used by gemmologists to discern between similar appearing gems. Specific gravity/ relative density can be defined as the weight of a body compared with the weight of an equal volume of pure water at 4oC, where density is defined as a measure of mass of a substance per unit volume. A substance weighing five times as much as an equal volume of water will have a SG of 5. Density directly relates to chemical composition and atomic/molecular structure and packing.

    The two most common methods of determining SG are hydrostatic weighing and heavy liquids. It should be noted here that even these methods are generally only used by professional gemmologists as they are not particularly portable, cheap or can use hazardous components, the theory however is always good to know.

    Specific gravity can be determined using the hydrostatic method, where a gemstone is first weigh in air and then totally immersed in liquid. SG is then found using the following equation:

    SG  =       W1         X  SG of liquid used
               W1 – W2
    Where: W1= weight in air
               W2  = weight in liquid
              SG of liquid used =  SG of pure water at 4oC (SG of 1)

     

    For example:
    Weight in air = 1.456g
    Weight in water =1.095g
    SG  =       W1         X  SG of liquid used
               W1 – W2
     SG  =        1.456g         X  1 (water)
               1.456g – 1.095g
            =   1.456g 
                 0.361g
     therefore:
        SG = 4.033

    sg-scales

    The heavy liquid method involves immersing the stone in liquids of different know SG and observing if the stone sinks, floats or sits in between the surface and the bottom f the container. If the stone has a higher SG  it will sink, if it has the same SG as the liquid it will hang somewhere in the middle and if it has a lower SG than the liquid it will float to the surface.

    The heavy liquid method uses the following liquids:

    • Bromoform and monobromonaphthalene; toluol- quartz (SG 2.65)
    • Pure Bromoform (SG 2.88)
    • Pure Methylene iodide (SG 3.32)
    • Dilute clerici solution spinel (SG 3.60)
    • Dilute clerici solution corundum (SG 4.00)

    heavy-liquids

    Gemstones with the same or similar SG’s to the heavy liquids above are:

    • Bromoform and monobromonaphthalene: quartz
    • Pure Bromoform: beryl, tourmaline.
    • Pure Methylene iodide: peridot, jadeite.
    • Dilute clerici solution spinel: diamond, topaz, spinel.
    • Dilute clerici solution corundum: ruby, sapphire.

     

    There are certain disadvantages and safety precautions which should be taken into account when using heavy liquids:

    • Bromoform and methylene iodide need to be kept out of direct light to prevent discolouration.
    • All should be regarded as poisonous.
    • All should be regarded as corrosive.
    • All must be cleaned off any surface immediately.
    • Contamination of the liquids with each other can cause major errors.

     

    As mentioned above these techniques are not usually carried out by anyone other than gemmologists, but an extreme example of the theory behind measuring density would be comparing three possible diamonds of the exact same size and shape. Take a glass imitation of SG 2 (range of glass is 2.0-4.2), a diamond of SG 3.52 and a cubic zirconia of SG 6 (range of zirconia is 5.5-6.0) and if the stones are large enough you may be able to heft the stones and physically feel the difference in weight between the exact same sized stones. If you can weigh the stones you can also get closer to separating them, a typical 1ct round brilliant cut diamond is about 6.4mm in diameter, so if you have a stone this size and it weighs closer to 2ct then you might have a cubic zirconia.

    For comprehensive lists of SG just have a troll through Google and you should find what you need to get you on your way.

  • The Strength of Gemstones

    Oval.Sapphire.Repair1-2

    As previously mentioned a key aspect of a gemstone is that it has sufficient durability. There are three aspects that make up how we assess durability; these are hardness, toughness and stability. It is important for jewellers and gemmologists to know about durability as this can affect how a gem is used, for instance you might not want to put turquoise into an open setting to be worn as a ring as this could easily lead to damage. You must also know the difference between each of the factors as even though emerald is quite hard it is not very tough, so similar care as taken with something like turquoise can also apply to emerald. It is also important in the formation of gem placer deposits, the more durable a gemstone the more likely it is to survive being rolled down a river without breaking up completely (good examples of this are diamonds and jade).
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  • Crystal Theory Part 2

    In the last article we looked at symmetry and as an extra note we should mention that integral symmetry of a crystal is not always obvious from its outward appearance even if the crystal is fairly well formed, it may appear squashed or have uneven side lengths. You will see this in the last diagram from part 1 and yet as also shown in this diagram you can see that the angles between the faces is constant. Thus it is usually more efficient to look at the angles between crystal faces than the faces themselves (at least in the beginning).

    axes
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  • Crystal Theory Part 1

    A crystal structure is a regular, repeating, three dimensional arrangement of bonded atoms. Though the generic term crystal is used to describe a single, solid crystal with defined crystal faces, much gem material is still crystalline even if its outward appearance is rough and irregular. This is because the internal structure within the material is still crystalline.

    aquamarine-amongst-tourmaline-rods-(mozambique)
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  • The Building Blocks of Gemstones: Extra Notes

    ameth-atom

    Ionic Bonding- bonding resulting from electrostatic attraction of negatively and positively charged anions and cations.
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  • The Building Blocks of Gemstones

    The world as a whole can be said to be a collection of atoms, the smallest divisible part of an element. Solid matter consists of bonded atoms and when these atoms are in an orderly, repeating structure they are described as crystalline. This property is extremely important in the world of gemmology as you will come to learn.
    In our last article we learned that “A mineral is defined as “a solid inorganic substance of natural occurrence” and they have a defined chemical composition and structure.” This also means that a mineral is crystalline. Most gems have a crystalline nature and are made up of minerals or aggregates of mineral, diamond being an example of a single mineral (and a single element) and Lapis Lazuli is an aggregate.

    dia-lapis Diamond Crystal and Lapis Lazuli

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  • Where do Gemstones Come From? Extra Notes

    rose-quartz-mineRose Quartz Mine, Cabo Delgado, Mozambique.

    Terms:

    Attached- Attached crystals are those which grow ‘attached’ to another rock an hence can only have one termination etc. Eg. Diopside, pyrite, quartz.

    In-situ- In the place of formation, e.g. emerald and tourmaline are often found in-situ.

    Disseminated- Disseminated crystals grow dispersed within other rocks and are commonly anhedral due to growth restriction. Eg. diamond, garnet, ruby.

    Gem gravel- Gem occurrences in gravels, clays, and other loose deposits derived by weathering of earlier rocks. Eg. sapphire, spinel, zircon.
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  • Where do Gemstones Come From?

    A mineral is defined as “a solid inorganic substance of natural occurrence” and they have a defined chemical composition and structure. There are over 3000 known minerals and although there are over 90 separate elements known, just eight of them make up around 99% of the Earth’s crust. As previously mentioned the Earth is in a constantly active state with convection below the crust driving mountain building, volcanoes, earthquakes and more and it is these earth processes along with erosion that create gem deposits.

    earth-inside
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  • A Good Place to Start, Extra Notes.

    Gemstone: A material possessing beauty, rarity and durability (and acceptability).

    e.g. diamond, ruby, emerald.
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  • A Good Place to Start

    Throughout history gems have fascinated and been coveted by human society, but just what makes a gem? A gem must be beautiful, rare and durable. A new factor becoming more apparent, especially in the eyes of such associations as Gem-A is acceptability. Any material that meets these criteria can be a gem whether it is a faceted diamond or even carved green plastic imitating jade.
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