We may also apply this hypothesis to the gems, and perhaps maintain that they too have arrived at perfection by progressive stages. The corundum, for instance, in the primitive rocks is never so pure and perfect as the nodules and crystals found in the true gem strata of recent formations. The emerald of the limestone is also incomparably above the beryls of the granites. The spinels, the chrysoberyls, the zircons, and the topazes of the gem beds are generally far superior to those found in the old crystalline rocks. There are, however, some plain exceptions to this plausible theory; and the finest of the tourmalines are found in cavities in granite ledges that appear to be of an early age. We are also sometimes inclined to think that color in the early ages of terrestrial life was wanting in the rich hues which now deck animate nature. For of all the relics of the old geologic forms that are preserved to us their colors are either greatly faded or were at first faintly painted. Even in the tertiary division the hues are not beautiful. The shells, however, exhibit a trace of the pearly hue of the nacre, which may once have shone as brightly as in the modern mollusca. Some of the fossil fishes display a gleam of the silver tints that now glisten on the sides of the living species. Fossil corals preserved in the marble, however, have retained the beauty of form but lost all delicacy of hue, if they ever possessed any. Still, absence of bright and glowing colors of the animals in a fossil state is by no means conclusive evidence that nature was then devoid of external decoration. For we may see on every side how the beautiful hues of animal and vegetable life may fade and disappear altogether on the suspension of vital activity; and also how the process of solidification and petrifaction may modify or even obliterate all traces of organic color. It is, however, a fact that the richest-colored gems and minerals are found near the surface of the earth, as though they required the direct influence of the solar rays, like the finest varieties of colored coral and the gorgeous flowers of vegetation.