CAUSE OF COLOR IN THE OPAL. Thus the opal is distinguished by the prismatic colors that emerge from it owing to the effect of thin layers of material of slightly different density, and hence of different refractive index from the rest of the material. These thin films act much as do soap-bubble films, to interfere with light of certain wave lengths, but to reflect certain other wave lengths and hence certain colors. Again, in some sapphires and rubies are found minute, probably hollow, tube-like cavities, arranged in three sets in the same positions as the transverse axes of the hexagonal crystal. The surfaces of these tubes reflect light so as to produce a six-pointed star effect, especially when the stone is properly cut to a high, round cabochon form, whose base is parallel to the successive layers of tubes. STARSTONES, MOONSTONES, CAT'S-EYES. In the moonstone we have another sort of effect, this time due to the presence of hosts of small twin crystal layers that reflect light so as to produce a sort of moonlight-on-the-water appearance _within_ the stone when the latter is properly cut, with the layers of twin crystals parallel to its base. Ceylon-cut moonstones are frequently cut to save weight, and may have to be recut to properly place the layers so that the effect may be seen equally over all parts of the stone, as set. Cat's-eye and tiger's-eye owe their peculiar appearance to the presence, within them, of many fine, parallel, silky fibers. The quartz cat's-eye was probably once an asbestos-like mineral, whose soft fibers were replaced by quartz in solution, and the latter, while giving its hardness to the new mineral, also took up the fibrous arrangement of the original material. The true chrysoberyl cat's-eye also has a somewhat similar fibrous or perhaps tubular structure. Such stones, when cut _en cabochon_, show a thin sharp line of light running across the center of the stone (when properly cut with the base parallel to the fibers). This is due to reflection of light from the surfaces of the parallel fibers. The line of light runs perpendicularly to the fibers. In these cases (opals, starstones, moonstones, and cat's-eyes) the individual stone is usually easily distinguished from other kinds of stones by its peculiar behavior towards light. However, it must be remembered that other species than corundum furnish starstones (amethyst and other varieties of quartz, for example), so that it does not follow that any starstone is a corundum gem. Also the more valuable chrysoberyl cat's-eye may be confused with the cheaper quartz cat's-eye unless one is well acquainted with the respective appearances of the two varieties. Whenever there is any doubt other tests should be applied. For further account of luster and other types of reflection effects see _Gem-Stones_, by G. F. Herbert-Smith, Chapter V., pp. 37-39, or _A Handbook of Precious Stones_, M. D. Rothschild, pp. 17, 18. LESSON VIII HARDNESS