Difference between revisions of "Specific Gravity"

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The specific gravity (SG) of gemstones is a constant 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.<br />
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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.<br />
 
The method of determination uses a [[Hydrostatic Balance|hydrostatic balance]].
 
The method of determination uses a [[Hydrostatic Balance|hydrostatic balance]].
  
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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).
 
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).
  
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).
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Since 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===
  
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.  
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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.
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. In gemology, g/cm³ is used.
 
  
 
===Mass and weight===
 
===Mass and weight===
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The method of measuring SG is with a hydrostatic balance.<br />
 
The method of measuring SG is with a hydrostatic balance.<br />
First, the stone is weighed in air and then weighed when fully immersed in water.
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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.
 
 
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>
 
:<math> SG = \frac{weight\ of\ stone\ in\ air}{weight\ of\ stone\ in\ air\ -\ weight\ of\ stone\ in\ water}</math>

Revision as of 11:46, 28 October 2006

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.

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

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