This test is also called Mohs’ scale of hardness test. This is a simple test in which the hardness of the test is based on the hardness of the surface. The surface of the stone is scratched with the help of a pen knife, and the hardness is gauged based on the relative abrasiveness of minerals (the soft being talc and the hardest being diamond). Such a classification is given by Mohs and the scale is divided into the ten following scales:
- Talc
- Gypsum
- Calcite
- Fluorspar
- Apatite
- Orthoclase feldspar
- Quartz
- Topaz
- Corundum (saphire)
- Diamond
Thus, if a mark is made by the pen knife on the surface of the stone, the hardness may be equated to that of a calcite and hence the hardness, H = 3. A siliceous rock surface can not be scratched which may be equated to that of quartz and in this instance, H = 7.
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