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One of the outstanding properties of most gem stones is hardness, for often it is only by this merit that they have survived their long journey from mountain heights to valleys and river beds. In general, gem stones are amongst the hardest of nature’s products, and diamond takes pride of place, being the hardest substance known.
By hardness, we mean the resistance which a stone offers to the separation of its particles. The degree of hardness varies considerably, and with certain exceptions, the same stone will always exhibit the same degree of hardness. This degree can be measured relatively by scratching two minerals together, and hardness points, which consist of a graduated series of different minerals of varying degrees of hardness, are sometimes used. Fragments of these minerals are so set that they can be conveniently applied to the surface of a given specimen. If a mineral definitely scratches another, it is naturally the harder of the two, but this is an elementary test and one on which no great reliance can be placed.
A more exact instrument was devised by Brinell, a Swedish scientist. By using his method, the relative hardness of any substance can be tested, and the Brinell hardness number is now generally recognized, particularly when metals and alloys are being considered.
There are other hardness testers, such as the Rockwell machine and the Vickers diamond pyramid tester, but all are based on the same principle, that is, a diamond point is pressed or revolved on a surface for a given time under a definite load or pressure, and from the nature and area of the indentation or abrasion made the hardness number can be calculated.
A scale of hardness was drawn up by the mineralogist Mohs, and this is still in general use with gem stones, although it only shows relative hardnesses with no degree of equality between the different grades. Diamond is given the hardness of 10, and other stones to denote the lower degrees are
9    Corundum.
8    Topaz.
7    Quartz.
6    Orthoclase.
5    Apatite.
4    Fluor spar.
3 Calcite.  2Gypsum.  1Talc.
The hardness of most gem stones, according to this scale, is given in the Appendix, but it must be clearly understood that the figures in this scale have no quantitative significance. In other words, diamond with a degree of 10 is not just twice as hard as apatite, which is rated at 5. It is, in fact, very many more times as hard, the actual difference in hardness between diamond and corundum (ruby or sapphire) being far greater than that between corundum and talc. Diamond is far and away the hardest of gem stones.
Gypsum, a variety of which is alabaster, is so soft that it may be scratched with a finger nail. Window glass is about 5, a pen-knife will scratch minerals up to about 6 1/2 and the finger nail will mark stones having a hardness of about 2 1/2. Each stone given in the list of hardnesses will be scratched by a stone of higher degree.