JADE. The term jade readily evokes the concept of a hard and precious, semitranslucent green stone. However, not only does jade appear in a wide variety of colors, such as white, brown, black, green, and even purple, but the term also describes two quite distinct stones, nephrite and jadeite. Nephrite, the stone of ancient China, is a silicate of calcium and magnesium and exhibits a felted, fibrous structure resembling wood grain as well as a soft, waxy luster. Jadeite, on the other hand, a pyroxene silicate of aluminum and sodium, has a cryptocrystalline structure, giving it an often grainy appearance. It is not only harder than nephrite, polishing to a glasslike finish, but it also appears in a wider variety of colors, including emerald green as well as rich blues and purples. Jade is universally admired for its beauty and durability as a precious stone, for its inexhaustible...