George Frederick Herbert Smith

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.

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.

Delicate tests show that the structure of all crystallized substances is more or less grained, like that of wood, and the hardness for the same stone varies in different directions. Kyanite is unique in this respect, since its hardness ranges from 5 to 7; it can therefore be scratched by a knife in some directions, but not in others. In most substances, however, the range is so small as to be quite imperceptible. Slight variation is also apparent in the hardness of different specimens of the same species. The diamonds from Borneo and New South Wales are so distinctly harder than those from South Africa and other localities that, when first discovered, some difficulty was experienced in cutting them. Again, lapidaries find that while Ceylon sapphires are harder than rubies, Kashmir sapphires are softer.

Hardness is a character of fundamental importance in a stone intended for ornamental wear, since upon it depends the durability of the polish and brilliancy. Ordinary dust is largely composed of grains of sand, which is quartz in a minute form, and a gem-stone should therefore be at least as hard as that. Paste imitations are little harder than 5, and consequently, as experience shows, their polish does not survive a few weeks’ wear. Hardness is, however, of little use as a discriminative test except for distinguishing between topaz or harder stone and paste. Diamond is so much harder than other stones that it will leave a cut in glass quite different from the scratch of even corundum. Paste, being so soft, readily yields to the file, and is thus easily distinguished from genuine stones. In applying the test to a cut stone, it is best to remove it from its mount and try the effect on the girdle, because any scratch would be concealed afterwards by the setting. Any mark should be rubbed with the finger to assure that it is not due to powder from the scratching agent; confusion may often be caused in this way when the two substances are of nearly the same hardness.

The degrees of hardness of the gem-stones are given in Table VIII at the end of the book.

It must not be overlooked that extreme hardness is compatible with cleavability in certain directions intimately connected with the crystalline structure; the property, in fact, characterizes many mineral species of different degrees of hardness. Diamond can be split in four directions parallel to the faces of the regular octahedron, a property utilized by the lapidary for shaping a stone previous to cutting it. Topaz cleaves with considerable ease at right angles to the principal crystallographic axis. Felspar has two directions of cleavage nearly at right angles to one another. The new gem-stone, kunzite, needs cautious handling owing to the facility with which it splits in two directions mutually inclined at about 70°.

All stones are more or less brittle, and will be fractured by a sufficiently violent blow, but the irregular surface of a fracture cannot be mistaken for the brilliant flat surface given by a cleavage. The cleavage is by no means induced with equal facility in the species mentioned above. A considerable effort is required to split diamond, but in the case of topaz or kunzite incipient cleavage in the shape of flaws may be started if the stone be merely dropped on to a hard floor.