HARDNESS
The scale of hardness does not represent exact and absolute degrees of that quality, but is simply an arrangement of minerals of different degrees from the softest to the hardest known, numbered, for convenient reference, from 1 to 10. Many suppose the numbers represent exact quantities in the progressive scale, as the numbers of inches on a foot-rule express an exact quantity of extension. It is not so. Minerals, constant in the quality of hardness, of various degrees, were selected, and numbers attached to them as they increased in hardness, from talc, the softest, to diamond, the hardest; but the difference between the minerals does not necessarily correspond with the ratio of the numbers. The difference between the diamond (10) and sapphire (9) is much greater than between sapphire (9) and topaz (8). In fact, the diamond is said to be as much harder than the sapphire as the sapphire is harder than talc. The scale is arranged simply as a means for the comparison of the relative hardness of minerals, and not for the purpose of expressing in terms a definite degree of the quality. In saying that emerald is 7.5 to 7.8, it is meant that its hardness lies about half-way between that of quartz (7) and topaz (8), and that its variations incline towards the harder mineral. The Moh scale is as follows:
1. Talc, common foliated variety.
2. Gypsum, or rock salt.
3. Calcite, transparent variety.
4. Fluorspar, crystallized variety.
5. Apatite, transparent crystal.
6. Feldspar, cleavable variety.
7. Quartz, transparent variety.
8. Topaz, transparent crystal.
9. Sapphire, cleavable variety. 10. Diamond.
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