CATALOGUE OF GEMS AND PRECIOUS STONES. 83 



structure, but which are so dense as to appear almost amorphous. 

 (See under Chalcedony and Opal.) 



Composition. — Essentially silicon dioxide, SiO,; some varieties 

 contain admixed iron oxides. 



Crystallization. — -Hexagonal (trigonal), trapezohedral ; crystal habit 

 usually prismatic, with pyramidal terminations (fig. 9) ; some va- 

 rieties, such as chalcedony, are only known in the cryptocrystalline 

 condition. 



Color. — Colorless when pure, but many of its important varieties 

 are based on colors due to the presence of impurities. 



Luster. — Vitreous; in some varieties, waxy; transparent to opaque. 



Hardness. — 7; very durable. 



Specific gravity. — 2.66 when pure, but varying considerably in 

 impure forms. 



Crystallized quartz has a mean refractive index of 1.55 and is 

 optically uniaxial and positive. The chief means of its ready deter- 

 mination are, however, its hardness, which is such that it will scratch 

 glass, its lack of cleavage, and general glasslike appearance and 

 fracture. It is one of the most common of minerals. Small crystals 

 have been produced by artificial means, but never in sizes or colors to 

 rival the natural material. Quartz and the cryptocrystalline varieties 

 of silica are much used in the cheaper grades of jewelry, and it can be 

 scarcely regarded as more than a semiprecious stone, although the 

 better grades of amethyst are very beautiful. The other varieties 

 are worth little more than the cost of cutting. (See further under 

 Chalcedony.) 



The chief commercial source of amethyst used in jewelry is the 

 Department of Artigas in northwestern Uruguay and adjacent parts 

 of Brazil. A great deal of the material is exported to Germany, cut, 

 and distributed throughout the world. Cut stones of the material are 

 to be found in all of the leading towns of the United States, and are 

 often sold to the unwary public as a local product. The large 

 botryoidal mass of crystals shown in a special case at the end of the 

 mineral hall is stated to have come from a gigantic geode found in 

 1900 in the Province of Rio Grande do Sul, Brazil. 



LIST OP SPECIMENS. 



QUARTZ. 



SILESIA. 



Two double cabochon, circular girdle; colorless, opalescent; 11.28 and 10.30 



carats; 13.5 by 10 and 13 by 9 mm No. 1410 



Brilliant, circular girdle; pale yellow, opalescent; 2.41 carats; 9.5by5mm.. No. 1411 



(JOTTED STATES. 



Maine. 



Paris, Oxford County: 



Cabochon, rectangular girdle; white, opalescent; 103.26 carats; 44 by 23 

 by 12 mm No. 1832 



