Hardness

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Revision as of 06:35, 19 November 2006 by Doos (talk | contribs) (Directional 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 abrasion 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.

As can be seen in the list, quartz has a hardness of 7 on Mohs' scale. Quartz is one of the most abundant minerals on Earth and is even in dust particles that float through the air constantly. Therefor minerals with a hardness less than 7 will abrase relatively rapid with normal wear.

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.

Directional hardness

Due to various causes, some minerals are harder in one direction than in another direction. Kyanite has a hardness of 5 in the basal plane direction, while it has a hardness of 7 in the prismatic directions. This phenomenon (varying hardness in different directions) is named "directional hardness".
Diamond also has directional hardness but not to the extend of Kyanite. Diamond is softer in the octahedral planes and harder in the cubic planes (and even softer in the dodecahedral planes). This property helps the diamond cutter as diamond dust which is orientated in a random fashion will always have part of the dust aligned in its hardest direction and can easily saw and polish the diamond in the octahedral direction of the to-be-cut diamond.

Directional hardness is also named "differential hardness".