A precious stone is one highly prized for human adornment. Its value is measured mainly by its beauty, its rarity, and its durability. The precious stones are, by general understanding, limited to the diamond, the ruby, the emerald, and the sapphire. The pearl, though strictly not a stone and far less durable, is nevertheless, because of its beauty and the rarity of superb specimens, included among the precious gems. A gem is a precious stone of rare quality, especially when cut and polished. All other stones used in jewelry are semiprecious. _Light on the Stones_ Stones may be characterized according to their response to light. Lustrous stones are those which catch the light brilliantly and glow almost as though with an inner flame. The cutting of the gem may aid in this effect, as with the diamond. Vitreous stones are of the glassy type, not lustrous. These may be transparent, permitting one to see objects clearly through the stone, like a fine crystal; or they may be translucent, permitting one to see light and shadow but not distinct objects through the stone. When light falls upon such translucent stones as moss agate, moonstone and agate, there is a soft glow. Or stones may be opaque, permitting no passage of light, like the turquoise. Because of their crystalline structure, even the opaque stones, however, may respond glowingly to light. Sometimes when the stone was formed, tiny cracks or bubbles stayed between the crystals. As the stone is moved, these cracks cause a play of prismatic colors which seems almost the sparking of an inner fire. To the names of such stones the term _fire_ is prefixed; they are extremely rare and beautiful. The Empress Josephine had a fire opal so remarkably aflame that she called it “The Burning of Troy.” _Star Gems_ Another variation from regularity, which can scarcely be called an imperfection or a flaw, enhances the beauty and the value of a precious stone. A certain break or tiny space in the crystalline structure may produce a radiation of three lines crossing at a single point, giving the effect of a six-pointed star. The star ruby and the star sapphire are among the most highly prized of all gems. _The Pearl_ The pearl has been described as “a disease of the oyster.” A tiny foreign object, such as a grain of sand or a chip off the inside of the shell (this inside is called nacre or mother-of-pearl) becomes imbedded in the oyster itself; it is, of course, an irritant. Drawing upon its natural resources but unable to expel the foreigner, the oyster protects itself by isolating the intruder, building around the speck a thin layer of an iridescent fluid, similar to that which lines the oyster shell. This fluid hardens, layer after layer. Given proper time—about four or five years—and the proper species of oyster—not the kind commonly used for food—and a pearl is born.