Specific Gravity

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The specific gravity (SG) of gemstones are constants widely used in gemological property charts. Although not every gemologist enjoys doing a SG test, it is still a property which can be very useful when other general tests fail.
The method of determination uses a hydrostatic balance.

Basic

Specific gravity (also known as "relative density") is the ratio between the weight of a stone in air and the weight of an equal volume in water. By convention, the temperature of the water is 4° C and at standard atmosphere, as the density of water is greatest under these conditions. Room temperature conditions are adequate for gemological purposes, as the small difference in density of the water will have little effect on the readings (measured to the second decimal).

As specific gravity is relative to the mass (or weight) in air and the mass in water, it is a ratio and isn't expressed in units (such as kg/m³). For instance, the SG of Diamond = 3.52 (whereas the density of Diamond = 3.52 g/cm³).

Density

Density is different from specific gravity in that it is the mass of an object divided by its volume, expressed in kg/m³ by SI (Le Système International d'Unités - The International System of Units) standards. In gemology, g/cm³ is used.

Other weighing systems are still widely in use (mostly in the USA and the UK), but the metric system of the SI is slowly finding its way there as well.

Mass and weight

Mass is the amount of material in an object and is a physical property of that object (like a gemstone), expressed in kg (kilogram) by SI standards.

Weight is the gravitational force (9.8 m/s²) on that object and is expressed in N (newton). Weight is not a physical property as it may change under different situations. A stone would weigh less on the Moon than on Earth, while the mass would remain the same.

As can be concluded, we should use "mass" instead of "weight" when being scientifically correct, but in daily use mass and weight are interchangeable.

The carat (ct) is an accepted unit of mass (or weight, if you please).

Measurement of specific gravity

The method of measuring is with a hydrostatic balance.
First, the stone is weighed in air and then weighed when fully immersed in water.

After this, SG is calculated through a simple formula.

<math> SG = \frac{weight\ of\ stone\ in\ air}{weight\ of\ stone\ in\ air\ -\ weight\ of\ stone\ in\ water}</math>


A demonstration can be seen in this video.


Video.png Specific Gravity Video
Video showing the method of determining hydrostatic specific gravity - WMV/video format - 7.96MB

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