541 



MINERALOGV. 



Orycto^no- prisms which are variously bevelled and acuminated. 

 . _ *? ^ The lateral planes of the crystals are longitudinally 

 strealted ; but the acuminating and bevelling planes 

 are smooth ; the terminal planes rough. Externally 

 splendent ; internally, splendent and vitreous. Frac- 

 turo small nnd perfect conchoidal. Alternates from 

 transparent to semitransparent ; and refracts double. 



Chemical Characters. Saxon topaz in a gentle heat 

 becomes white, but a strong heat deprives it of lustre 

 ;md transparency: the Brazilian, on the contrary, by 

 xposiire to a high temperature, burns rose-red, and in 

 a still higher violet-blue. 



Physical Characters. When heated, exhibits at one 

 extremity positive, and at the other negative electricity. 

 It also becomes electrical by friction, and retains this 

 property for a considerable time, sometimes more than 

 twenty-four hours. Brazilian Topaz. 



Const. Paris. Alumina, . 58.38 



Silica, . 34.01 



Fluoric acid, . 7.79 100.18 



Berzclius, Af handlingar, vol. iv. p. 23S. 



Geognostic Situation. It occurs in various primitive 

 rocks, such as topaz-rock, gneiss, and clay-slate. 



Geographic Situation. It occurs in large crystals, 

 and rolled masses, in an alluvial soil, in the granite and 

 gneiss districts of Mar and Cairngorm, in the upper 

 jwrts of Aberdeenshire ; and in veins, along with tin- 

 stone, in clay-slate, at St. Anne's, in Cornwall ; also in 

 St. Michael's Mount, and at Trevaunance, in the ssmc 

 county. The finest topazes are those found in Brazil. 



Uses. This gem is much prized by jewellers, and is 

 considered as one of the more beautiful ornamental 

 stones. 



SECOND SUBSPECIES. 



Schorlite. ScHORLiTO, or SciioHLous TOPAZ, Jameson. Schor- 

 lartiger Berill, or Piknit, Werner, P3'ciiite, Haiij. 



External Characters. Colours yellow, white, and 

 grey. Occurs almost always massive, also in pa- 

 rallel, thin, and straight prismatic distinct concre- 

 tions, which are longitudinally streaked, and crystal- 

 lized in long six-sided prisms, which are sometimes 

 truncated on the terminal edges and angles, and are 

 generally imbedded. Externally and internally its lus- 

 tre is shining, approaching to glistening, and is resi- 

 nous. Fracture small and imperfect conchoidal, or 

 fine-grained uneven. More or less translucent on the 

 edges. Brittle. Uncommonly easily frangible. 



Geognostic and Geographic Situations. It occurs at 

 Altenberg in Saxony, in a rock of quartz and mica 

 which forms an imbedded mass, included in porphyry. 



THIRD SUBSPECIES. 



riiysalite. PHYSAUTE, or PVROPIIYSALITE, Jameson. Pyro- 

 physalith, Hisinger, Physalith, Werner. 



'External Characters. Colours greenish-white and 

 mountain-green. Massive. Occurs in coarse granular 

 distinct concretions. Lustre of the cleavage splendent, 

 of the cross fracture glistening or dull. Fracture un- 

 even or conchoidal. Translucent on the edges. 



Geographic Situation. It is found imbedded in gra- 

 nite at Finbo, near Fahlun, in Sweden. 



. GENUS VI. EMERALD. 



Rhomboidal. Prismatic. Cleavage prismato-rhom- 

 boidal, prismatoidal ; first axifrangible. Hardness 7 r >, 

 8.0. Sp. gr. = 2.6, 3.2. 



1. PRISMATIC EMERALD, or EUCLASE, Jameson. 

 Euclas, Werner. Prismatischer Smaragd, Mohs. 



Prism = 133 26'. Cleavage prismatoidal, in the di- 

 rection of the smaller diagonal. Hardness = 7.5. Sp. 

 gr. = 2.9, 3.2. 



External Characters.-~-Co\ours, white, green, and 

 blue. Crystallized in oblique-four sided prisms various- 

 ly modified by truncations., and bevelrneirts, and acu 





minations. Internally splendent. Fracture small 'con- 

 choidal. Alternates from transparent to translucent, 

 and refracts double. 



Chemical Characters. Ketore the blowpipe, it first 

 loses its transparency, and then melts into a white enamel. 

 Const. Parts. Silica, . 35 

 Alumina, 18 



Glucina, . 1 J. 

 Iron, . 2 



Loss, . 31300. I'auijuclin. 



Geagnoslic and Geographic Situation, This rare and 

 beautiful mineral occurs in Peru and Brazil. 



'2. RHOMDOIDVL EMERALD, Jameson. Rhomboed- 

 rischer Smaragd, Mo/n. dul Emer- 



Di-rhomboid= 138 35'; 90. Cleavage most dis- '<' 

 tinct parallel with the terminal planes of the six-sided 

 prism, less distinct in the direction of the lateral planes 

 of the six-sided prism. Hardness = 7.5, 8.0. Sp.gr. 

 = 2.6, 2.8 



FIRST SUBSPECIES, 



PRECIOUS EMERALD, Jameson. Schmaragd, Werner. Precious 

 External Characters. Colour emerald green, gene- Emerald, 

 rally crystallized in equiangular six-sided prisms. La- 

 teral planes smooth ; terminal planes rough. Inter- 

 nally lustre intermediate between shining and splen- 

 dent, and vitreous. Fracture small and imperfect con- 

 choidal. Alternates from transparent to translucent, 

 ant! refracts double in a moderate degree. 

 Const. Parts. Silica, . 64.5 



Alumina, . 16 



Glucina, . 13 



Oxide of Chrome, 3.25 

 Lime, . 1.6 



Water, . 2.0100.35 



J-'atiguelin, Jour, des Mines, N. 38. p. f)S; 

 Geognostic Situation. It occurs in drusy cavities, in 

 veins in clay -slate, and also imbedded in mica-slate; and 

 loose in the sand of rivers and other alluvial deposites. 

 Geographic Situation. The most beautiful emeralds 

 are at present brought from Peru ; and others of less 

 value are found in Salzburg. 



Use. It is rare to find the colour of this gem pure 

 and of good strength ; hence such specimens are very 

 highly valued, and are employed in the most expensive 

 kinds of jewellery. It is valued next to the ruby ; and 

 when of good colour, is set without a foil, and upon a 

 black ground, like brilliant diamonds. 



SECOND SUBSPECIES. 



BERYL, Jameson. Edljer Beril, Werner. Bern. 



External Characters Co]aurs green, blue, and yel- 

 low. Occurs massive, and this variety sometimes 

 appears arranged in straight and thin prismatic dis- 

 tinct concretions. Often crystallized in long equi- 

 angular six-sided prisms, either perfect, or trunca- 

 ted on the lateral and terminal edges or angles, also 

 sometimes acuminated. Lateral planes deeply longitu- 

 dinally streaked, but the terminal, acuminating, and 

 truncating planes are smooth. Externally, lustre shin- 

 ing and glistening ; internally shining, which sometimes 

 passes into glistening and splendent, and vitreous. 

 Fracture small, and more or less perfect conchoidal. 

 Commonly transparent, and refracts double, but in a 

 feeble degree. 



Const. Parts. Silica, . 69.50 



Alumina, . 14.00 



Glucina, . 14.00 



Oxide of Iron, . 1.0098.50 



Rose, in Karsten's Tabcllen. 



Geognostic Situation. It occurs in veins that traverse 

 granite and gneiss, also imbedded in granite, and dis- 

 persed through alluvial soil. 



Geographic Situation. It occurs in alluvialsoil along 



