# Specific Gravity

The specific gravity (SG) of gemstones is a constant widely used in gemological property charts. Although not every gemologist enjoys doing an 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.

## Contents

## 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 because 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).

Since specific gravity is relative to the weight of an object in air and its weight 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).

In gemology, specific gravity is, usually, determined through an apparatus based on Archimedes' Principle.

Archimedes' Principle (or the Law of Buoyancy) states that: the upward force on an immersed object is equal to the weight of the displaced fluid.

This may sound complicated but it is a fairly simple, yet brilliant, law.

Consider two balls of equal weight but of different specific gravity, for instance a 10 gram gold ball and a 10 gram silver ball. The gold ball has a sg of 19.3, while silver has a sg of 10.5.

Because gold has a sg that is almost twice as much as silver, you can imagine that the 10 gram gold ball will be smaller than the 10 gram silver ball. In other words, the gold ball will have a smaller volume than the silver ball.

When you hang both balls in water (immersed), then the silver ball will displace much more water than the golden ball due to its higher volume.

Water has a sg of 1, so the weight of a cubic centimeter of water is 1 gram (actually 0.0098 Newton, but grams used for simplicity). Through some simple math we can calulate the volume of the balls. The volume of the gold ball is 0.52 cubic centimeter and the volume of the silver ball is 0.95 cubic centimeter (volume is mass divided by density).

From this we can conclude that the silver ball will displace 0.95 cubic centimeter of water, which weighs 0.95 gram. The golden ball will displace 0.52 grams of water (because 1 cubic centimeter of water weighs 1 gram).

Now back to Archimedes' Principle: the upward force on an immersed object is equal to the weight of the displaced fluid (the fluid being water in this case). The silver ball displaces a higher weight of water, so it will experience a larger upward force than the golden ball and will rise higher in the water when immersed.

A common mistake is to drop the object in the water in a way that it will sink to the bottom. It can not work in that case as it is then no longer "immersed".

### 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 Earth's moon than on Earth, but its 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 SG is with a hydrostatic balance.

First, the stone is weighed in air and then weighed when fully immersed in water. After this, the weights are inserted into 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 presentation

Specific Gravity Video (Hosted by Google Video) |

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