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Chapter 7 Notes.docx

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Earth Sciences
Jianping Xu

Chapter 7 Notes Definitions  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 4 C o 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
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