Serpentine has been largely used for decorative purposes, as well as for cameos and intaglios, and formed most of the famous ‘verde antique.’ Being the result of the decomposition of other silicates it varies enormously in appearance and characters, but the most attractive stones are a rich oil-green in colour and resemble jade. The composition approximates to the formula H_{4}Mg_{3}Si_{2}O_{9}, but it invariably contains other elements. The hardness varies from 2½ to 4 on Mohs’s scale, according to the minerals contained in the stone; the specific gravity is about 2·60 and the refractivity 1·570. The beautiful rose-red stone, thulite, makes a handsome decorative stone. It has nearly the same composition as epidote (p. 275), and like it has strong dichroism, the principal colours being yellow, light rose, and deep rose. The colour is due to manganese. Its refractive index is about 1·70, specific gravity 3·12, and hardness 6 to 6½ on Mohs’s scale; it possesses an easy cleavage. Fine specimens come from Telemark, Norway, and it is therefore called after the old name for Norway, Thule. Marble is a massive calcite, carbonate of lime, with the formula CaCO_{3}. When pure it is white, but it is usually streaked with other substances which impart a pleasing variety to its appearance. It is always readily recognized by the immediate effervescence set up when touched with a drop of acid. Calcite is highly doubly refractive (cf. p. 40), the extraordinary index being 1·486, and ordinary 1·658, a difference of 0·172; the specific gravity is 2·71, and hardness 3 on Mohs’s scale. Lumachelle, or fire-marble, is a limestone containing shells from which a brilliant, fire-like chatoyancy is emitted when light is reflected at the proper angle. It sometimes resembles opal-matrix, but is easily distinguished by its lower hardness and by its effervescent action with acid. Choice specimens come from Bleiberg in Carinthia, and from Astrakhan. Apophyllite has not many characters to commend it, being at the best faintly pinkish in colour, and always imperfectly transparent. It is a hydrous silicate of potassium and calcium with the complex formula (H,K)_{2}Ca(SiO_{3})_{2}.H_{2}O. Its refractivity is about 1·535, specific gravity 2·5, and hardness 4½ on Mohs’s scale; it possesses an easy cleavage. It occurs in the form of tetragonal crystals at Andreasberg in the Harz Mountains, and in the Syhadree Mountains, Bombay, India. PART II—SECTION D ORGANIC PRODUCTS CHAPTER XL PEARL, CORAL, AMBER