What is specific gravity? Archimedes discovered that the weight of body in air minus its weight in water is equivalent to the weight of the water displaced by the body. Specific gravity is defined as "The weight of a body compared with the weight of an equal amount of pure water at 4°C (4°C is the temperature at which water is densest). When a body is placed in water, the volume of water displaced is equal to the volume of the body. When the body is placed in water that it undergoes an apparent loss of weight. This loss of weight is equal to the weight of the water displaced.

We'll substitute the word a "body" to a "gem stone" to explain specific gravity although you can really determine the specific gravity of any material.

When a gem stone is weighed in air and then weighed in water the loss of weight is equal to the weight of its volume in water displaced. The weight of the gem in air divided by the loss of weight in water gives the specific gravity of the stone.

Typically room temperature distilled water is used. It is best to use water that is boiled, because deionized water that has been sitting for a time to degas water and lessen the chance of fine air bubbles. A tiny drop of wetting agent, alcohol, or dishwashing liquid maybe added to the water to decrease surface tension of water which can adversely affect the results. One drop of detergent liquid the size of a pin head destroys surface tension in 1 liter of water.

The specific gravity of a diamond is 3.52. This mean the weight of diamond is 3.52 times that of an equal volume of water. There are a number of ways to write the equation for the specific gravity (SG). Here is the most common:

(Weight in Air) x (SG of Fluid)

SG = ---------------------------------------------

Loss of Weight in Water

Since the specific gravity for distilled water at 4°C is 1 the equation can be rewritten as...

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