Difference between revisions of "Hardness"

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Hardness of a gemstone is one of the components that defines the durability of the gemstone (others are toughness, brittleness and stability - resistance to heat and/or chemicals). In gemology we define hardness as the ability to withstand abbrasion by other materials.
 
Hardness of a gemstone is one of the components that defines the durability of the gemstone (others are toughness, brittleness and stability - resistance to heat and/or chemicals). In gemology we define hardness as the ability to withstand abbrasion by other materials.

Revision as of 06:09, 19 November 2006

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--Doos 05:09, 19 November 2006 (PST)

Hardness of a gemstone is one of the components that defines the durability of the gemstone (others are toughness, brittleness and stability - resistance to heat and/or chemicals). In gemology we define hardness as the ability to withstand abbrasion by other materials.

Basic

Mohs' hardness scale

Mohs' scale of relative hardness
1. Talc
2 Gypsum
3. Calcite
4. Fluorspar
5. Apatite
6. Orthoclase feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond

In the early 19th century Friedrich Mohs developed a scale of relative hardness for minerals. He composed a list of 10 minerals with known hardness and organized them so that they are on an ascending scale, with talc at the lower end and diamond at the highest end.
This scale indicates that corundum can scratch topaz (and all others lower on the list) but not diamond.

It should be noted that this is not a linear scale. The difference in actual hardness between diamond and corundum is 10 times higher than the difference between talc and corundum. Therefor it is a comparitive hardness scale and the hardness of one mineral is relative to another mineral on the list.

Of course all minerals could be put on the scale, yet that would create an almost endless list.

The method of testing the hardness of gemstones is by the use of hardness pencils.

Hardness pencils

Hardness pencils are pencils with a tip made of one of the 10 minerals on Mohs' scale.
These pencils are used to try and scratch a stone and observe if the pencils can scratch the gem. As this is a destructive method (it leaves a scratch), this test is mostly used on pieces of rough or on a insuspicable place of the gemstone. Not many gemologists use this test as there are better ways to determine a gemstones identity.

When one does feel the need to apply this test, one must do it in a logical manner.
The gemstone is tested with a pencil of low hardness (starting at 4 or lower) and one tries to put a tiny scratch on the stone to be tested. When there is no scratch observed, one repeats this process with a pencil of higher hardness (in this case 5) and so on until one arrives at the pencil that is able to put a tiny scratch on the stone.
For instance if one could not scratch the mineral with a pencil no.7 but it would leave a scratch with pencil no.8, the hardness of the mineral must be between 7 and 8 on Mohs' scale.