In examining the interior of an opal, we often fail to perceive any cause for the reflections of color, especially in the limpid varieties. The flashes appear when the light enters the stone at a certain angle, but when viewed in any other direction the gem presents the usual appearance of common transparent quartz. In other varieties of the mineral, however, especially the milky or translucent, a cause for the colored reflections is easily observed. We have under observation the beautiful opal known as the “Oberon,” and beneath its translucent surface appear thin films of a faint reddish hue suspended at different depths within its interior. They are so well defined that their edges may be recognized, and they lie like thin clouds suspended in a hazy sky. But as the gem is turned so that the light strikes the film at a different angle, the scene is instantly changed, and a mass of flame replaces the sombre tint. It is a little curious that some of the films exhibit the different colors of the spectrum as the angle of light is changed, while others display only the green and blue color, no matter how the light strikes them. In fact, the films or patches, which are apparently alike, produce different results from the same rays of light; and some display the continuous spectrum, while others exhibit but one color. It is a little singular that all transparent minerals when fractured do not exhibit alike in their fractures the prismatic gleams displayed by quartz and glass. We have before us a beautiful transparent white crystal of adularia or moon-stone from St. Gothard, and although it is fissured and fractured in a thousand places, yet we observe little iridescence in it even when exposed to the sunlight. The localities where the precious opal is now found are but few, and none of them were probably known to the ancients. All record of the old opal mines is now lost; but there were undoubtedly deposits of the mineral in Arabia, Syria, and in Asia, whence the ancients derived their gems. The famous Hungarian mines were not discovered until late in the fifteenth century, and the country was quite unknown to the Romans.