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.

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