528 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



Aragonite. — This mineral, like calcite, consists of calcium carbonate (CaCOg), but 

 differs from the latter in crystalline form and in physical characters. It also occurs 

 sometimes in finely fibrous aggregates, when it cannot be distinguished on mere inspection 

 from finely fibrous calcite, for which in its varied uses it may be substituted. The 

 calcareous deposits (Spriidelstein) of the hot springs of Carlsbad in Bohemia also consist of 

 aragonite; this material is often banded in red, white, and brown, and is fashioned into all 

 kinds of small articles, which are sold as souvenirs to visitors at the baths. 



Gypsum. — This mineral also occurs like calcite, but more frequently, in fibrous veins, 

 the fibres of which are perpendicular to the walls of the veins. When the mineral has a 

 very finely fibrous structure it exhibits a fine satiny lustre, and may be cut for ornaments of 

 various kinds just as calcite is. The hardness is still less, however (H = 2), and the stone 

 can be scratched even with the finger-nail. Material of this character is afiundant in the 

 extensive deposits of gypsum, largely quarried for the manufacture of plaster of Paris, in 

 the Triassic rocks of Derbyshire and Nottinghamshire. Massive gypsum with a granular 

 structure is known as alabaster, but, like marble, is never used as a gem. 



FLUOR-SPAR. 



Fluor-spar or fluorite is much too soft for use as a precious stone, but notwithstanding 

 this is sometimes worn as a ring-stone or other ornament for the sake of its fine colour. It 

 is often substituted for more valuable precious stones which it happens to resemble in 

 colour, but its chief use is as a material for ornamental objects of various kinds. When 

 perfectly clear and colourless fluor-spar has an important application in the construction of 

 apochromatic lenses, and the mineral has several other technical applications. 



Fluor-spar is an abundant and widely distributed mineral. It occurs in a massive and 

 compact condition, filling veins and crevices in rocks, and is associated with barytes and 

 metallic ores, especially of lead. Magnificent groups of regularly developed crystals 

 resting on the matrix are sometimes found in cavities in these veins, the crystals being 

 finely coloured and transparent enough for use as gems. The finest specimens are found in 

 England, in the counties of Derbyshire, Cumberland, Cornwall, and Devonshire, the two 

 first named being most famed for these crystals. Good specimens occur also in the 

 metalliferous veins of the Harz, in the Erzgebirge, and in the Black Forest. The mineral 

 is so widely distributed that it is only possible here to enumerate the most important 

 localities. 



In its purest condition fluor-spar contains 4872 per cent, of calcium and 51-28 per 

 cent, of fluorine, and is thus fluoride of calcium with the formula CaFg. The mineral 

 frequently occurs in fine crystals belonging to the cubic system. The cube is the form 

 most commonly assumed, but the octahedron and other simple forms occur alone or in 

 combination with each other. Twin crystals are not uncommon, t^\■o cubes inter- 

 penetrating in such a way that they are symmetrical with respect to each other about a 

 face of the octahedron. Massive crystalline aggregates of granular or columnar structure, 

 and compact masses of use only for technical purposes, are also common. 



Fluor-spar has a perfect cleavage in four directions parallel to the faces of the 



