EARTHSC 2M03 Lecture Notes - Hydrostatic Weighing, Gemstone, Relative Density

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Chapter 7 Notes
The SG (specific gravity) of a substance is the ratio of its weight of an identical volume of
pure water
Because SG is expressed as a ratio, no units of measurement are necessary
The SG of diamond for example is 3.52, which indicates that diamond is 3,52 times heavier
than an identical volume of water
The density of a substance is defined as its weight per unit volume, and is measured not as a
ratio but in units of weight and volume
Relative density is a term sometimes used in connection with liquids, and like SG is the ratio
between the weight of the liquid and the weight of an identical volume of water at 4oC
Archimedes’ Principle and the Measurement of SG
Archimedes’ principle states that a body immersed in a fluid experiences an upward force
equal to the weight of the fluid it displaces
As the value of SG is a reliable constant for the majority of gemstones, it can be a useful aid
in the identification of an unknown (and unmounted) specimen
The SG of a substance depends in part on the atomic weights of its constituent elements, and
in part on the compactness of the structure formed by these elements
Finding the stone’s precise weight is not difficult, but unless the specimen happens to be
perfect cube or similar regular shape, the calculation of its volume could be complicated
Approximation of SG Using Heavy Liquids
This method, although less precise than hydrostatic weighing method, has the advantage that
it is fast and that there is no bottom size limit to the specimens
In its simplest form it consists of 4 liquids having specific gravities between 2.65 and 3.32
The gemstone under test is immersed in each liquid in turn, and its SG lies between that of
the liquid in which it sinks and that in which floats on the surface
The most important of the surviving liquids is the organic fluid compound di-iodomethance,
CH2I2 (methylene iodide)
Containers for heavy liquids should be wide-necked for the easy insertion and retrieval of
gems, and of clear glass so that the gem can be seen without difficulty during the test
Practical Points for SG Measurement Using Heavy Liquids
1) Hold the stone in tweezers, and insert it just below the surface of the liquid before releasing
2) If the stone sinks in the liquid, observe the rate of sinking. If the stone only sinks slowly, or
rises slowly to the surface, its SG will be very close to that of the liquid.
3) If the stone immediately bobs to the surface after being released, its SG is significantly less
than that of the liquid.
4) Always clean both the gemstone and the tweezers before transferring the stone from one
liquid to another
5) As it is easier to retrieve a gemstone which is floating, start the test using the 3.32 liquid. If
the stone floats in this, go to the next highest liquid and if necessary work downwards until
the stone sinks.
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