Page:Handbook of Precious Stones.djvu/21



properties of precious stones as are perceptible to the eye unaided by optical apparatus, but trained to keenness of vision, afford valuable means of discriminating precious stones from one another, but do not exhaust such means. Indeed, such mechanical properties as hardness and specific gravity are of the greatest use in determining the species of a stone, and are more commonly available than the majority of optical tests. Optical properties must, however, ever hold a chief place in all artistic classifications of precious stones, so that it will not be unadvisable to begin the present chapter by a synopsis of the most obvious characters of this class. They may be arranged in the form of a tabular view, the use of which is twofold, enabling us to define the several optical properties found in gem-stones, and also to appreciate their artistic capabilities. We arrange these optical (or mainly optical) qualities under the general heads of "Surface" and "Substance:"