Mineral Guide > Gems > Hardness of Gems

Hardness of Gems

Tests of hardness afford one of the most useful and convenient means of distinguishing gems. Such tests can be easily made and are very reliable, the hardness of species being remarkably constant. Hardness should not, however, be confounded with toughness, i. e., the difficulty with which a mineral can be broken, since many brittle minerals have considerable hardness. Hardness is rather the power of resistance to scratching which a mineral possesses.

It is evident that a high degree of hardness must be an important property of precious stones, as their polish would soon disappear if they were easily scratched.

The common method of stating the hardness of a mineral is by referring it to its place in the scale devised by the German mineralogist Mohs. The divisions of this scale are constituted by ten rather common minerals, arranged according
to their hardness.

The scale is as follows:

1. Talc.
2. Gypsum.
3. Calcite.
4. Fluorite.
5. Apatite.
6. Feldspar.
7. Quartz.
8. Topaz.
9. Corundum.
10. Diamond.

To assist in remembering the minerals of this scale in their order, the following mnemonic has been devised:

Tale Gypsum Calcite Fluorite Apatite Feldspar Quartz Topaz Corundum Diamond

The position of a mineral in this scale is determined by the minerals which it scratches. Thus if a mineral scratches feldspar, but is scratched by quartz, its hardness would be stated as 6.5. In order to test hardness, pieces of the minerals of the scale should be at hand. Fragments of the mineral to be tested may be grasped in the fingers and rubbed upon a polished surface of the minerals of the scale, or the test can often be more accurately made by rubbing upon the mineral of the scale a coarse powder of the mineral to be tested, by means of a soft pine stick.

With a little practice one may become so good a judge of the hardness of a mineral, by its behavior towards an ordinary pocket-knife, that the minerals of the scale below 7 may be dispensed with. Thus minerals of the first two degrees of hardness may be scratched with the finger nail; \ o. 3 can be deeply scratched with a knife; No. 4 less deeply and easily; No. 5 still less so; while 'NO. 6 is about the hardness of the knife. No. 6 also scratches ordinary window glass. Upon No. 7 a knife blade makes no impression, the steel rubbing off on the mineral. Steel of the hardness of a file, however, scratches quartz slightly. These tests are especially useful for distinguishing glass imitations from gems of the hardness of quartz and higher. Instead of a file it is well to use a point of hardened steel to avoid danger of injuring delicate gems. Rubbing the gem, especially if cut, with an aluminum pencil, is a still better means of testing hardness in the higher numbers of the scale, as it involves no danger of injury to the stone. Upon soft stones such a pencil leaves a conspicuous mark, but upon hard ones none whatever. Minerals above 7 in hardness are harder than a file. Corundum scratches all minerals except diamond, and diamond is the hardest substance known.

Some minerals, if crystallized, are somewhat harder in one direction than another, the mineral cyanite being a notable illustration of this.