# Specific Gravity

The specific gravity (SG) of gemstones are constants widely used in gemmology 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 is with 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 the largest 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 (we measure at the 2nd 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 (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 aswell.

### Mass and weight

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

Weight is the gravitational force (m/s²) on that object and expressed in N (newton). This is not a physical property as it may change under different situations. A stone would experience a larger gravitational force on the top of the Eiffel Tower than on the ground.

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

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

### Measurement of specific gravity

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

After this the SG can be calculated through a simple formula.

$SG = \frac{weight\ of\ stone\ in\ air}{weight\ of\ stone\ in\ air\ -\ weight\ of\ stone\ in\ water}$

A demonstration can be seen in this video.

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