To estimate the value of a rough diamond and ascertain its purity is often a difficult task, and one that requires both education and tact. For the surface of the natural gem, especially the modified and spheroidal crystals, is generally dull and chatoyant. This singular appearance, which has been erroneously represented as a thin crust, is in reality produced by the salient edges of the laminæ of which the stone is composed. The same or a similar effect may be artificially produced in the polished diamond by heating it to a white heat, as has been seen in the diamonds injured in the great fires of Hamburg and Chicago. Barbot, the French jeweller, declared that he had discovered a means by which the apparent rough, translucent coating could be changed into perfect transparency so as to give a clear view of the condition of the interior. This statement, however, is very much doubted by lapidaries. When the rough diamond is placed in turpentine, alcohol, or even water, it appears more transparent than before and like a bubble of air. We have therefore sometimes thought that Barbot had really discovered some fluid of high refractive power, immersed in which the rough diamond became transparent. For it is a well-established fact known among chemists, that rough gems of a lower refractive power, like sapphire, chrysoberyl, spinel, etc., if placed in a fluid possessing an equal refractive energy, like that of muriate of antimony, become clear, and the observer is enabled to look through them. This discovery strengthens Barbot’s statement and places it within the possibilities; but as yet we know of no fluid that possesses a refractive power equal to the diamond. Topaz, emerald, and other gems of low refractive power can be readily examined by using the oil of cassia, which has the refractive energy of 1.64.