CATALOGUE OF GEMS AND PRECIOUS STONES. 55 



2. Iron garnet: Andradite. — Calcium-iron garnet. 



3. Chromium garnet: Ouvarovite. — Calcium-chromium garnet. 

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



of 3.55 to 3.66, and a considerable color range. The several varieties 

 are: Essonite (cinnamon stone or hyacinth), of which the specimens 

 of a clear yellow-brown to deep gold tinged with brown are more 

 commonly used as gems. Grossularite includes the pale green, yel- 

 low to nearly white, pale pink, red-orange, and brown kinds. Iloman- 

 zovite is a brown variety; wiluite is yellow-green to greenish white; 

 topazolite is deep to pale yellow; and succinite is amber-colored. 



The principal magnesian garnet is the pyrope, meaning "'fire- 

 like," a deep red to nearly black stone, prized as a gem. It is among 

 the hardest of the garnets, ranking 7.5 in the scale. Its specific 

 gravity lies between 3.7 and 3.8. 



The almandite, or carbuncle, and rhodolite are iron-aluminum 

 garnets. Almandite varies in color from bright red to deep red of 

 several tints, occasionally assuming an orange hue by artificial light. 

 The color of the rhodolite lies between a violet-purple and a brown- 

 red. These varieties have a hardness of about 7.5, with a specific 

 gravity seldom less than 4, and occasionally as high as 4.3. Both 

 are prized as gems. 



Spessartite is a manganese-aluminum garnet, varying in specific 

 gravity from 3.7 to 4.3, and has a hardness of about 7. The color 

 varies from a red-brown, sometimes with a tinge of violet, to orange 

 red. It often affords fine gems. 



The calcium-iron garnet varies in specific gravity between 3.6 and 

 4 and in hardness from 5 to 7. The group includes a diversity 

 of forms, varying widely in color and other respects, the more impor- 

 tant of which are: Andradite, a yellow or orange-brown variety; 

 demantoid, or Uralian emerald, a grass-green, emerald-green, or 

 brown-green stone having a brilliant luster, and when cut exhibiting 

 considerable fire, especially by artificial light; colophonite, a brown- 

 black garnet, characterized by a resinous luster; and melanite, a 

 black to yellow-brown kind. 



The calcium-chromium garnet, ouvarovite, is almost invariably a 

 fine emerald green color, and is harder than any of the other varieties, 

 ranking nearly 8 in the scale. 



Garnet is common in mica, hornblende, and chlorite schist, 

 gneiss and granite, occurring also in limestone, serpentine, and vol- 

 canic rocks (fig. 6). The garnet of granite, gneiss, mica schist, 

 and similar rocks is commonly almandite. Grossularite is com- 

 mon in limestones and crystalline schists. Pyrope belongs espe- 

 cially to peridotites and the serpentines derived from them; occurs 

 also in basalts. Spessartite occurs in granitic rocks, in quartzite, 

 in certain schists, and in some rhyolites. Iron garnets are com- 

 mon in eruptive rocks, occurring also as a product of contact meta- 



