The Viennese
mineralogist Friedrich Mohs (1773-1839) invented the scratch
hardness test. He defined scratch hardness as the resistance
of a mineral when scratched with a pointed testing object.
Mohs chose ten minerals of different hardness for comparison
and graded these minerals one to ten. Each mineral in this
series scratches the previous one. Minerals of equal
hardness cannot scratch each other. By comparative
application of Mohs' hardness scale the hardness (according
to Mohs) of every gem can be determined. Stones with scratch
hardness 1 and 2 are soft, 3 to 6 medium hard, over 6 hard.
Minerals of Mohs' hardness 8 to 10 are also described as
"hard gems." The luster and polish of gems of hardness below
7 can be damaged by dust as this may contain small particles
of quartz (Mohs hardness 7). Such stones must be carefully
handled when worn or stored so that they do not come into
contact with any scratching objects.
Relative
and Absolute Hardness Scale
Scratch
hardness
(Mohs) |
Mineral
used for
comparison |
Simple
hardness tester |
Cutting
hardness
(Rosiwal) |
|
1 |
Talc |
Can be
scratched with fingernail |
0.03 |
2 |
Gypsum |
Can be
scratched with fingernail |
1.25 |
3 |
Calcite |
Can be
scratched with copper coin |
4.5 |
4 |
Fluorite |
Easily
scratched with knife |
5.0 |
5 |
Apatite |
Can be
scratched with knife |
6.5 |
6 |
Orthoclase |
Can be
scratched with steel file |
37.0 |
7 |
Quartz |
Scratches window glass |
120.0 |
8 |
Topaz |
Scratches window glass |
175.0 |
9 |
Corundum |
Scratches window glass |
1,000.0 |
10 |
Diamond |
Scratches window glass |
140,000.0 |
|