54 o THE POPULAR SCIENCE MONTHLY. 



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 en- 

 hances 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 glass- 

 like, 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 de- 

 creases, 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 ad- 

 mixtures. 



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 cer- 

 tain that this formation from liquids into solid bodies has taken place 

 in a large class of half-precious stones, such as quartz and pyrites, con- 

 sisting likewise of nothing but silica namely, agate, jasper, opal, chal- 

 cedony, chrysoprase, carnelian, heliotrope, and others. 



At the same meeting of the Parisian Academy where MM. Fre- 

 my 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 



