Some gems are of a different color by artificial as compared with day light. The beauty of some may thus be enhanced by artificial light and that of others weakened. The gem in which the most striking change is thus produced is chrysoberyl of the variety known as alexandrite. This is green by daylight, but red by artificial light. Most yellow stones appear nearly colorless by artificial light because the excess of yellow rays in the latter makes those from the stone almost invisible. For the same reason violet stones are likely to lose much of their color in artificial light. One of the points of superiority of the emerald is that it is able to retain its color in all lights. The color of the ruby is deepened and made more brilliant by artificial light, and turquois of good color has its effect enhanced by the same.