Precious stones are, on the basis of their relative transparency, divided into four classes, as follows: _Transparent_, or admitting light freely and clearly; defining objects when used as a lens. _Semi-transparent_, admitting light, but only partially defining objects. _Translucent_, admitting light faintly. _Opaque_, not admitting light. The more valuable precious stones, excepting opals and turquoises, are generally transparent. PHOSPHORESCENCE. Some precious stones display a distinct phosphorescence after exposure to the sunlight, and also upon the application of artificial heat, and through mechanical and electrical means. Many diamonds, when taken to a dark room, appear quite luminous; this is also true of topaz, fluor spar, and other minerals. ELECTRICITY. Minerals acquire electricity through friction or heating, and in this state readily attract or repel small bits of paper and other light substances. All minerals are electric, some displaying positive and others negative electricity. The electric test of a precious stone refers to the length of time that a stone will retain electricity after friction or heating. Some stones lose this quality in a few minutes, while others retain it a long time. The tourmaline is noted for its electrical properties, while the Brazilian topaz rendered electric by heating or rubbing has been known to affect the electric needle after 32 hours. CUTTING AND POLISHING. Although a finely developed diamond, ruby, or other crystal is sometimes found and used for jewelry, the beauty of a precious stone generally remains hidden within a rough and unsightly exterior until the lapidary’s art reveals the gem. According to well known rules, there is one kind of cutting or faceting for the diamond or colorless gems and another for colored gems. The brilliant cut, figs. 5 and 6, consists of an arrangement of fifty-six facets, exclusive of the table and culet. This cut is sometimes improved by the addition of eight star facets around the culet, which brings the number of facets up to sixty-four. The following are the proportions of a well cut diamond or colorless gem: ⅓ above the girdle, fig. 6, A. ⅔ below " " " 6, B. The table 2/5 of the breadth of the stone, fig. 6, C. The culet ⅙ of the size of the table, fig. 6, D. [Illustration: FIG. 5. FIG. 6.] These proportions do not refer to colored gems, which are cut thick or shallow to deepen or diminish the color of the stone. The step cut, fig. 7, now principally used for emeralds, can be advantageously used for other colored stones. The crowned rose cut, fig. 8, is applied to small diamonds, and occasionally to colored gems. This cut consists of twenty-four facets, and a well proportioned rose is one half of its diameter in thickness. [Illustration: FIG. 7. FIG. 8.] [Illustration: FIG. 9. FIG. 10.] To the smaller and more common roses only twelve facets are given. Besides the above-mentioned forms, there are the: