By the term _luster_ we refer to the manner and degree in which light is reflected from the _surface_ of a material. Surfaces of the same material, but of varying degrees of smoothness would, of course, vary in the vividness of their luster, but the type of variation that may be made use of to help distinguish gems, depends upon the character of the material more than upon the degree of smoothness of its surface. Just as silk has so typical a luster that we speak of it as silky luster, and just as pearl has a pearly luster, so certain gems have peculiar and characteristic luster. The diamond gives us a good example. Most diamond dealers distinguish between real and imitation diamonds at a glance by the character of the luster. That is the chief, and perhaps the only property, that they rely upon for deciding the genuineness of a diamond, and they are fairly safe in so doing, for, with the exception of certain artificially decolorized zircons, no gem stone is likely to deceive one who is familiar with the luster of the diamond. It is not to be denied that a fine white zircon, when finely cut, may deceive even one who is familiar with diamonds. The author has fooled many diamond experts with an especially fine zircon, for the luster of zircon does approach, though it hardly equals, that of the diamond. Rough zircons are frequently mistaken for diamonds by diamond prospectors, and even by pickers in the mines, so that some care should be exercised in any suspicious case, and one should not then rely solely on the luster. However, in most cases in the trade there is almost no chance of the unexpected presence of a zircon and the luster test is usually sufficient to distinguish the diamond. (Zircons are strongly doubly refractive, as was said in Lesson III. on Double Refraction, and with a lens the doubling of the back lines may be seen.) ADAMANTINE LUSTER. The luster of a diamond is called _adamantine_ (the adjective uses the Greek name for the stone itself). It is keen and cold and glittering, having a metallic suggestion. A very large per cent. of the light that falls upon the surface of a diamond at any low angle is reflected, hence the keenness of its luster. If a diamond and some other white stone, say a white sapphire, are held so as to reflect at the same time images of an incandescent light into the eye of the observer, such a direct comparison will serve to show that much more light comes to the eye from the diamond surface than from the sapphire surface. The image of the light filament, as seen from the diamond, is much keener than as seen from the sapphire. The same disparity would exist between the diamond and almost any other stone. Zircon comes nearest to having adamantine luster of any of the other gems. The green garnet that is called "olivine" in the trade also approaches diamond in luster, hence the name "demantoid," or diamond like, sometimes applied to it.