crust; and the diamond, which is the prince of all precious stones, is simply pure crystallized carbon, and so allied to charcoal, lampblack, etc. Other highly-esteemed precious stones, such as the emerald, the aqua-marina, and chrysoberyl, on the one hand, and the hyacinth, on the other, contain "earths" chemically related to argillaceous—earth namely, the former consist of beryl-earth, and the latter of zirconia; but these earths in themselves are neither rare nor precious, so that in some countries the streets are paved with the impurer brothers of the emerald. The same is true of all other precious stones, including pearls; in the main they are formed of substances of no value whatever, and to be found everywhere, such as argillaceous earth, silicic acid, fluor-spar, boracic acid, lime, magnesia, etc. Their only superiority consists in the fact that the common substance in them has reached an extraordinary degree of crystallization, for, aside from their beauty, their rarity enhances their value in the market.
Chemical combinations and simple substances of mineral as well as of organic nature assume their due crystal shapes, which are so well defined as frequently to bear a strong resemblance to those of cut stones, only when they pass from the liquid into the solid state, and they assume a large size only when this transition takes place very slowly. For instance, if we dissolve in hot water as much alum as can be dissolved therein, and suspend in the fluid, while allowing it to cool in a quiet place, a wire vessel—a basket, a rosette, or a crown, wrapped in wool—we shall find next morning that wire vessel covered with glasslike, transparent, more or less large, glittering octahedral crystals. Cold water is unable to hold in solution as large a quantity of the salt as warm water; and the surplus, as the temperature of the water decreases, has to separate slowly from it. In so doing, small crystals are formed. They grow constantly as the separation goes on, and, if we leave the solution exposed to the fresh air so that it slowly evaporates, we shall at last obtain very large crystals. If the alum contained an impure admixture of other salts, they would remain in the water. Crystallization, as a general thing, is also a purification of foreign admixtures.
In all probability, in Nature many precious stones have formed in the same manner; and most mineralogists concur in the opinion that rock-crystals, consisting of nothing but silicic acid, and frequently weighing hundreds of pounds, have originated thus. It is almost certain that this formation from liquids into solid bodies has taken place in a large class of half-precious stones, such as quartz and pyrites, consisting likewise of nothing but silica—namely, agate, jasper, opal, chalcedony, chrysoprase, carnelian, heliotrope, and others.
At the same meeting of the Parisian Academy where MM. Frémy and Feil described their process of manufacturing artificial rubies and sapphires, M. Monnier stated that he had obtained artificial opals by pouring a highly-diluted solution of oxalic acid cautiously upon a