Hydrostatic weighing is always available, unless the stone be very small, but the necessary weighings occupy considerable time, and care must be taken that no error creeps into the computation, simple though it be. Even if everything is at hand, a determination is scarcely possible under a quarter of an hour. The third method, which takes even longer, is intended primarily for powdered substances, and is not recommended for cut stones, unless there happen to be a number of tiny ones which are known to be exactly of the same kind. The specific gravities of the gem-stones are given in Table VII at the end of the book. CHAPTER IX HARDNESS AND CLEAVABILITY Every possessor of a diamond ring is aware that diamond easily scratches window-glass. If other stones were tried, it would be found that they also scratched glass, but not so readily, and, if the experiment were extended, it would be found that topaz scratches quartz, but is scratched by corundum, which in its turn yields to the all-powerful diamond. There is therefore considerable variation in the capacity of precious stones to resist abrasion, or, as it is usually termed, in their hardness. To simplify the mode of expressing this character the mineralogist Mohs about a century ago devised the following arbitrary scale, which is still in general use. MOHS’S SCALE OF HARDNESS 1. Talc 2. Gypsum 3. Calcite 4. Fluor 5. Apatite 6. Orthoclase 7. Quartz 8. Topaz 9. Corundum 10. Diamond A finger-nail scratches gypsum and softer substances. Ordinary window-glass is slightly softer than orthoclase, and a steel knife is slightly harder; a hardened file approaches quartz in hardness, and easily scratches glass. By saying that a stone has hardness 7 we merely mean that it will not scratch quartz, and quartz will not scratch it. The numbers indicate an order, and have no quantitative significance whatever. This is an important point about which mistakes are often made. We must not, for instance, suppose that diamond has twice the hardness of apatite. As a matter of fact, the interval between diamond and corundum is immensely greater than that between the latter and talc, the softest of mineral substances. Intermediate degrees of hardness are expressed by fractions. The number 8½ for chrysoberyl means that it scratches topaz as easily as it itself is scratched by corundum. Pyrope garnet is slightly harder than quartz, and its hardness is said therefore to be 7¼.