Not all stones that are doubly refracting exhibit dichroism. White stones of course cannot exhibit it even though doubly refracting, and some colored stones, though strongly doubly refracting, do not exhibit any noticeable dichroism. The zircon, for example, is strongly doubly refracting, but shows hardly any dichroism. The test is most useful for emerald, ruby, sapphire, tourmaline, kunzite and alexandrite, all of which show marked dichroism. It is of little use to give here the twin colors in each case as the shades differ with different specimens, according to their depth and type of color. The deeper tinted stones of any species show the effect more markedly than the lighter ones. The method is rapid and easy--it can be applied to mounted stones as well as to loose ones, and it cannot injure a stone. The student should, if possible, obtain the use of a dichroscope and practice with it on all sorts of stones. He should especially become expert in distinguishing between rubies, sapphires, and emeralds, and their imitations. The only imitation (scientific rubies and sapphires are not here classed as imitations), which is at all likely to deceive one who knows how to use the dichroscope is the emerald triplet, made with real (but pale) beryl above and below, with a thin strip of green glass between. As beryl is doubly refracting to a small degree, and dichroic, one might perhaps be deceived by such an imitation if not careful. However, the amount of dichroism would be less in such a case than in a true emerald of as deep a color. Those who wish to study further the subject of dichroism should see _Gem-Stones_, by G. F. Herbert-Smith, Chapter VII., pp. 53-59, or see _A Handbook of Precious Stones_, by M. D. Rothschild, Putnam's, pp. 14-16. LESSON V SPECIFIC GRAVITY The properties so far considered as serving to distinguish precious stones have all depended upon the behavior of the material toward light. These properties were considered first because they afford, to those acquainted with their use, very rapid and sure means of classifying precious stones. DENSITY OF MINERALS. We will next consider an equally certain test, which, however, requires rather more time, apparatus, and skill to apply.