It is a Myth that Specific Gravity of stone and Purity are proportionate.

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It is very commonly believed that higher Specific gravity means more purity.

IT IS WRONG

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In simple terms, Specific gravity compares the density of a mineral with the density of water.
If the density of a mineral is 3, it simply means that one unit of the mineral is 3 times heavier than one unit of water.

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The specific gravity of each gemstone is a fixed value. Because the 'heaviness' of a Gemstone is fixed, and heaviness does not define the quality.

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There is no question of higher or lower specific gravity, that may have an impact on quality.

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The Specific gravity of some gemstones are as follows

Sapphire: 3.99
Emerald: 2.72
Ruby: 3.99
Opal: 2.15

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For example, If the specific gravity of a sample stone is 4.5, then it is definitely NOT a Sapphire. Because we know that the specific gravity of sapphire is 3.99.

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This is how testing is done in laboratories. Properties like specific-gravity, Refractive Index, Optical Character, etc. are matched one by one until the result is confirmed.

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On the other hand,
If we compare multiple samples of sapphires, all sapphires would have the same specific gravity, even if they are of different quality.
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A sapphire of price $100 & another sapphire of price $5000 would have the same specific gravity.
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Therefore, Specific Gravity is an identification property.

It does not provide any information about the quality.
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The Quality of Gemstone is defined by:
Colour
Transparency
No. of inclusions.
Types of Inclusions
Cut Symmetry
Lustre
Surface quality
Presence of Defects
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While Buying a Gemstone, One must focus on quality parameters mentioned above instead of just looking at Specific Gravity.

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