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Thread: Introduction to Specific Gravity and Gemstone identification.

  1. #1
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    Default Introduction to Specific Gravity and Gemstone identification.

    Suppose somebody shows you a red stone, we can immediately say it is not an emerald (yes there are red beryls but lets stick to the classic definition, which is green), a peridot , or a tanzanite. But what is it? I could be many things, perhaps; ruby spinel, tourmaline, cubic zirconia or worse glass! After all the large Black Prince Ruby in the British Crown Jewels has proven not to be a ruby at all, but a large red spinel!

    Perhaps surprisingly, the process of Gemology is one of elimination rather than confirmation. By eliminating as many possibilities as possible, we can eventually draw a conclusion that a give stone must be of a give type.

    Archimedes was faced with the same problem in around 250BC, when King Hiero II asked him to prove his crown was gold and did not have silver or something mixed in. Archimedes discovered that he could measure the volume of an irregular shaped object by measuring the amount of water it displaced. From this and the weight he could calculate the density of the material, and compare it to a know sample.

    Today, we refer to this as specific gravity (SG), and have table for comparison, rather than specimens but the principals remain the same. I will leave out the additional physics and math’s, but by comparing the weight of a gemstone measured in air and again in water, we can calculate the SG

    SG = Weight of Gemstone / Wight Loss measured in water

    Whilst SG wont tell is that the stone is defiantly an xyz it will allow us to eliminate a large number of variables. Gemologists will measure a host of other properties such as the angle or angles a given stone bends light, or how it reacts to ultra violet light, to name a couple, but with SG we are a big step forward.

    Set stones pose a problem as weighing them will include the setting, but the following method will provide some reassurance when handling stones before setting, further this method uses equipment that a jeweler probably already has to hand, combined with a little DIY.

    Key to this method is a quality digital scale. I use a TANITA 1210, which is capable of measuring down to 0.002G or 0.01CT. This level of accuracy is desirable for small stones, but 0.01G should obtain workable results in all but the smallest cases, however the margin for error will increase accordingly. Ideally that the scale has a “TARE” or “Zero” Button. OK you could achieve the same result with addition subtraction, but quality equipment should have it anyway. Because mathematically we are looking at a ratio, the actual scale you use grams, ounces, carats etc. does not matter, provided both measurements are in the same scale! Personally I prefer to use grams.

    In addition to a scale you will need some for of stand and a harness to suspend the stone in water. The weigh of the harness can be “zeroed” out. Mine only weighs 4.5 grams! It is important that the harness should not touch the sides scale stand or the site of the water container. In the rig pictured below the scales is placed above the water further reducing the risk of water damage.

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    First measure the stone dry without the harness. This helps to keep water away from the scale!

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    Mount the harness on the scale and suspend the stone carrier in the water. Make sure you remove any bubbles on the stone carrier. A camelhair brush is useful for this.

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    Drop the stone in the water, and use the camelhair brush to remove any bubbles.

    zero the scale.

    Using a pair of tweezers, place the stone on to the stone carrier without taking it out of the water, and take the reading.

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    That’s it! All that is left is a bit of math’s; In this example I use a small purple stone

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    Weigh dry and in air: 0.210 grams.
    Weigh immersed in water: 0.130 grams.
    Wight loss: 0.210 - 0.130 = 0.08
    Specific Gravity = Weigh in air divided by weight loss: 0.210 / 0.08 = 2.65

    Amethyst is a type of quartz and 2.65 is consistent with quartz. A good gemologist wont jump to that conclusion yet with out further tests, but will eliminate a number of other possibilities eg purple CZ which would have an SG around 2.15 – 2.18. There are several other materials that have similar a SG to quartz

    coral 2.6-2.84
    feldspar 2.64
    scapolite 2.63
    soapstone 2.5 – 2.8
    turquoise 2.6-2.9

    With the exception of scapolite the others eliminate them selves by just looking at them. Scapolite can look similar to pale amethyst, so additional tests would be needed to eliminate this one.

    One particular of interest is testing diamonds. Naturally there are more tests for diamonds than any other stone! Most non-gemologists will test diamonds with an electronic thermal probe, but one imitation synthetic moissanite will fool probes that do not specifically test for it.. However its SG is 3.22 compared to diamonds 3.52. But beware white topaz has an SG of 3.56, it will however fail the probe. With the exception of topaz the SG of most common diamond imitations differ significantly. More recently we have seen improvement in diamond coating methods, these also could fool a probe, but should fail the SG test unless the SG of the core material is the same as diamond.

    Note: in a laboratory conditions the water should be 4 degrees centigrade and pure, however for practical purposes the cold tap should be sufficient only causing a variance in the third decimal place.
    Last edited by Julian; 11-04-2012 at 07:06 PM.

  2. #2
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    Thank you for that, Julian - I hadn't really thought of how to measure SG on stones, it's something I do with homebrew though...

    There was relatively recently a lot of interest in diamond film coated CZs, which were supposedly indistinguishable from the real thing. A friend of mine specialises in diamond vapour deposition, so was asked to run some tests using his equipment. Surprise - diamond film won't adhere to CZ; the example he'd been provided with showed no carbon in spectroscopy tests either. All that was being sold were very, very expensive CZs.

    I'd be very interested in seeing some more thoughts on gemstone identification/elimination - I did talk to Gem-A at IJL, but they didn't seem to have anything suitable as a short course to give greater confidence in stone ID.

  3. #3
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    It was WW2, and every week our class was made to recite a version of Archimedes' principle:

    'When a body is wholly or partly immersed in a fluid, it appears to lose weight and the loss is equal to the weight of the fluid displaced'.

    I had little idea of the implications, or that it would lead to this. Dennis.

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