Diamond is usually transparent, but it may be translucent and even opaque, especially the black varieties. Even otherwise transparent Diamond often contains inclusions which cloud and interrupt its clearness. These constitute the “flaws” which so often injure the value of a Diamond and prevent it from being of the “first water.” These inclusions may be simply small cavities, sometimes so numerous as to make the stone nearly black, or they may be particles of other minerals, such as chlorite, hematite or carbonaceous matter. If the latter, the flaws can sometimes be burned out by careful heating. As already remarked, the refractive power of the Diamond is very high. The rays of light entering it are bent at a high angle, causing a large degree of what is called total reflection within the stone. The effect of this is to light the stone’s interior. Moreover, the rays of light are concentrated on a smaller part of the surface than is the case with less highly refracting minerals and thus also internal illumination is produced. The most important result of the high refractive power of the Diamond is the wide dispersion of the spectrum, causing the red rays to be widely separated from the blue rays and strong lights of one color to be transmitted to the eye as could not be the case were the different rays less widely separated. It is this power of flashing different colored lights which gives the Diamond one of its chief charms. The index of refraction ranges from 2.40 for the red rays to 2.46 for the violet rays. Ordinary glass has an index of refraction for the red rays of only 1.52 and for the violet 1.54, making the spectrum only about half as long as that produced by the Diamond. Another pleasing property of the Diamond is the fact that it is usually more brilliant by artificial light than by natural, although some individual stones have a reverse behavior.