Identifying Minerals and Gems

Since many gemstones appear quite similar, which is why they can be simulated, it is important to be able to distinguish the real from the synthetic or from the simulant. If you have a scanning electron microscope (SEM) with XEDS available, the latter task is not a problem, but this is usually not the case in the field (or the shop). The basic tests are thermal conductivity, optical properties, and mechanical properties. Using a mechanical test to characterize a material that you do not want to damage is clearly tricky, so the use of hardness measurements is directed more toward minerals than gemstones. 

Indentation has been discussed in Section 16.3. Although these tests could be used for gemstones, they are not, except as a calibration. The Mohs scratch hardness scale is much more popular since the principle is to test what the stone will scratch, not what will scratch the stone. 

The Mohs hardness test (the word scratch is assumed) is almost a nondestructive test. The hardness of a gemstone is usually referred to as its Mohs hardness. Since this hardness value is determined by a scratch test, it is not actually a hardness. The scale has many drawbacks including the fact that it is not linear, it does not necessarily relate to wear resistance, and it damages the specimen so it is not ideal for polished stones. 

An additional problem in using mechanical tests for minerals is that many tend to cleave. Wear (or abrasion) might be a better test but it is more difficult for the gemologist. 

FIGURE 36.18 Pocket-sized instrument for measuring thermai conductivity. 

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