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 CaCO3. 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)2Ca(SiO3)2.H2O. 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
ALTHOUGH none of the substances considered in this chapter come within the strict definition of a stone, since they are directly the result of living agency, yet pearl at least cannot be denied the title of a gem. Both pearl and coral contain calcium carbonate in one or other of its crystallized forms, and both are gathered from the sea; but otherwise they have nothing in common. Amber is of vegetable origin, and is a very different substance.
Pearl
From that unrecorded day when some scantily clothed savage seeking for succulent food opened an oyster and found to his astonishment within its shell a delicate silvery pellet that shimmered in the light of a tropical sun, down to the present day, without intermission, pearl has held a place all its own in the rank of jewels. Though it be lacking in durability, its beauty cannot be disputed, and large examples, perfect in form and lustre, are sufficiently rare to tax the deepest purse.