The snow-white diamond displays the rainbow hues in the greatest perfection; and this is the reason why this quality is sought for in preference to the light buff or deeper yellow, which are in reality more brilliant. The deeper the hue of the gem, the less becomes the prismatic display; and when the diamond becomes of deep and decided hue, the colored reflections cease altogether. It is somewhat singular that the colored gems are generally more brilliant than the pure white, that is, if the color is not so deep as to affect the transparency of the stone. For examples, we shall find that the white sapphire has an index of refraction equal to 1.768, while the blue has 1.794, and the red 1.779. The refractive of the white topaz is 1.610, while the yellow is 1.632. The brilliancy and rainbow play of the diamond is not so apparent by daylight as by certain kinds of artificial light, when all its latent beauties are called forth as if by magic. The light of the camp-fire in the obscurity of night produces a marvellous effect upon the polished stone; and it is no wonder that the savage heart of the Russian General, Suvaroff, was fascinated by the vivid gleams of his treasured diamonds when viewed at night in the flickering beams of his bivouac fire. It may seem singular that the brilliant white light of gas does not display the qualities of the diamond as the duller flame of the wax candle. The secret lies, perhaps, in the difference in their spectra. Nevertheless, there is a great difference in their effects upon the gem, and it is a fact that the wax candle far exceeds the gaslight in calling forth the latent splendors of the gem. Therefore, we can assert that the brilliancy of toilets where the diamond is much worn depends greatly upon the manner of illuminating the apartment. We now come to another interesting problem in the study of the nature of the diamond. We refer to the various colors of the gem. As we have maintained that the mineral is of vegetable origin we may be expected to explain the phenomena of its color upon this hypothesis, and also account for the various changes of the gem when exposed to the effects of heat or the fire test. But we must admit with candor that our views concerning this physical property are decidedly unsatisfactory, and shall refer the reader to one of the chapters in our treatise on the Tourmaline, in which are grouped some of the theories relating to the subject. In fact, we may repeat the remarks of Huyghens, who said at the end of the seventeenth century: “In spite of the labors of Newton, no one has yet fully discovered the cause of the color of bodies.” “We must, then,” says M. Babinet, “admire, without penetrating their secret, the unparalleled red of the Oriental ruby, the pure yellow of the topaz, the unmingled greenness of the emerald, the soft blue of the sapphire, and the rich violet of the amethyst. This is not the only thing the discovery of which we shall leave to posterity.”