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Hardness, color and clarity scales

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From the Archives : Originally published August 21st, 2009
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Category : Gems and Treasures





What is the Mohs scale?


The Mohs scale measures the hardness or durability of minerals and natural gemstones.  Hardness refers to the gemstone’s ability to resist abrasion.  In other words, it describes the stone’s capacity to resist scratches from other minerals.  If you were to take a diamond (has the highest score) and try to scratch its surface with a ruby (the next highest score), you would be unsuccessful.  However, if you were to take the diamond and scratch the surface of a ruby, it would cause a scratch in the ruby stone. 


The actual rating


Ten different minerals or natural gemstones are graded on the Mohs scale, from one to ten, with ten being the highest.  The values, however, are only used as a standard because the differences between a natural gemstone that is ranked nine and one that is ranked ten is much greater than one that is ranked eight versus nine.   For instance, a diamond (a rank of ten) is roughly four times as hard as corundum (a rank of nine), while corundum is only twice as hard as topaz (a rank of eight). 


Listed below is the Mohs scale of hardness for minerals and natural gemstones:



Graphite – 0.5

Talc – 1
Gypsum – 2
Calcite – 3
Fluorite – 4
Apatite – 5
Feldspar – 6
Quartz – 7
Beryl (i.e. Emerald) – 7.5
Topaz – 8
Corundum (i.e. ruby, sapphire) – 9
Diamond – 10 


Mohs scale relative to other objects


It’s somewhat difficult to grasp the differences between the abstract numbers on the Mohs scale.  To put it into prospective, the following items have the corresponding hardness:


Lead in a pencil – 1
Fingernail – 2.5

Pearl - 2.5

Gold – 2.5 to 3
A penny – 3.5

Coral – 3 to 4

Platinum – 4 to 4.5

Tooth - 5
The blade of a knife – 5.5

Opal – 5.5

Lapis – 5 to 6

Turquoise – 5 to 6
Steel file – 6.5

Glass – 6 to 7

Garnet 7 to 7.5

Emerald – 7.5 to 8





A diamond works like a prism, dividing light into a spectrum of colours. The colours reflect in vivid flashes called "fire." Natural pigmentation in a diamond acts like a filter and diminishes the fire. The less colour in a diamond, the more varied the fire, and the better the colour grade. 





Diamonds that are absolutely clear are the most sought-after and therefore the most expensive. But virtually all diamonds have inclusions — scratches, trace minerals or other tiny characteristics that can detract from the pure beauty of the diamond, depending on how easily they can be detected.






All famous diamonds 




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