PRECIOUS STONES. 



PRECIOUS STONES. 



Most precious stones belong to such formations as gra- 

 nitic, gneissic, porphyritic rocks, &c., and are generally found 

 in the debris of such ; and although certain diamond-bearing 

 soils may be of a comparatively recent age, they are for all 

 that made up of the constituents of the older rocks. 



Corundum, sapphire, and ruby are found in gneiss, granite, 

 mica slate, chlorite slate, dolomite, or granular limestone. 



In Ceylon precious stones are searched for in the beds of 

 rivers, also in a gravel deposit (generally ten or twenty feet 

 below the surface). This deposit, called Nellan, consists of 

 waterworn pebbles, together with pieces of granite, gneiss, 

 &c. The gems occur in " pockets " and in groups. Eubies 

 are also found in dolomite. 



The Burmah rubies are found in a limestone deposit ; 

 also in alluvial deposits (formed from disintegrated gneiss 

 rock), in beds of rivers, in limestone rock, &c. 



Veins through mica-schist and clay-slate, black limestone, 

 cavities in granite have yielded emeralds : from porphyritic 

 rocks precious opal has been obtained, also from sandstone ; 

 also in a brown iron ore in Queensland. 



Emeralds are found in various metamorphic rocks : clay 

 slates, associated with calc spar, &c.* 



The turquoise of Persia is obtained from porphyritic tra- 

 chyte : that of Silesia and Saxony from clay-slate : that of 

 ,New Mexico in quartzite, sandstone, &c. In Arizona and 

 Nevada, too, turquoises are found in clay slate. 



Topazes are met with in talcose rocks, gneiss, granite, &c. 



Diamonds are usually met with in alluvial soil, often on 

 gold-diggings. In some Indian fields there is a diamond- 

 bearing conglomerate made up of rounded stones cemented 

 together, which lies under two layers, the top one of gravel, 

 sand, and loam, the bottom of thick black clay and mud. 

 Also found in flexible sandstone in America, India, &c. In 

 N.W. of West Australia in a gold-bearing conglomerate. 



In Brazil the most precious of all gems is obtained from 

 a conglomerate of white quartz, pebbles, and light-coloured 

 sand, sometimes with yellow and blue quartz and iron sand. 



* In Australia (with tinstone, topaz, fluorspar), in kaolin or decom- 

 posed granite, in North Carolina (North America), in a vein of quartz 

 and felspar, the country rock being mica-schiat. 



