CATALOGUE OF GEMS. 509 



brittle; has a hardness of -i, a specitie gravity of 3.18, and a highly 

 vitreous luster. The color range is extensive, including white, 3'ellow, 

 green, violet, sky and amethystine blue, brown, wine yellow, rose red, 

 crimson, and pink. It sometimes presents a ])luish fluorescence, and 

 is phosphorescent when heated gently. This is especially character- 

 istic of the variety chlorophane, or cobra stone, which emits a bright 

 emerald-green light on a comparatively low rise of temperature. 



Fluorite, though too soft for continuous wear, is occasionally cut as 

 a gem, chiefly for collectors' use, and a massive variety occurring in 

 the north of England is not infrequenth^ worked up into trinkets, 

 paper weights, vases, and other ornaments. 



Fluorite occurs in veins and beds in gneiss, mica slate, clay slate, 

 limestone, and sandstone. 



One hundred parts contain: Calcium, 51.3; fluorine, -18.7. 



GADOLINITE. 



Gadolinite, a complex silicate of yttrium and other earths, occurs 

 usually massive, rarely in monoclinic crystals. Its hardness is 6.5; 

 specific gravit3% 4.35; color, black, of several shades. The mineral 

 affords velvety black, opaque gems and is cut for collectors' use only. 



GARNET. 



The garnet includes several varieties, which are distinguishable by 

 their differences in composition and in part b}- their colors. The 

 mineral crystallizes in the isometric system, commonly in dodecahe- 

 drons and trisoctahedrons and derived forms. It is singly refracting 

 and monochroic; has a vitreous to resinous luster, and varies in hard- 

 ness and specific gravit3% according to the variety, from 6.5 to 7.5 and 

 3.15 to 4.3, respectively. 



In composition garnet is a silicate of different bases — alumina, lime, 

 magnesia, chrome, iron, manganese, and titanium. There are three 

 prominent groups, with several subdivisions under each, many of 

 these passing into each other. They are: 



I. Aluminum garnet: 



Grossularite — Lime-aluminum garnet. 

 Pyrope — Magnesium-aluminum garnet. 

 Almandite — Iron-aluminum garnet. 

 Spessartite — Manganese-aluminum garnet. 

 II. Iron garnet: 



Andradite — Lime-iron garnet. 

 III. Chromium garnet: 



Ouvarovite— Lime-chrome garnet. 



The lime-aluminum garnet has a hardness of 7, a specific gravity of 

 3.55 to 3.66, and its color range includes white, pale green, amber, 

 honey, wine, and brownish yellow, cinnamon brown, and pale rose red. 

 The several varieties are: Esf<onitc^ eijinamon intone, ov hyacinth. The 



