74 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



quartz. Tabasheer is an amorphous, opal-like silica deposited within 

 the joints of bamboo; it absorbs water and becomes transparent like 

 hydrophane. 



Opal occurs as a secondary deposit in fissures in many kinds of 

 rocks, also in petrified wood and other fossil material, being deposited 

 in the gelatinous form from solutions of silica, and hardening as 

 water is gradually expelled. Material of the composition of opal 

 can be readily produced artificially, but it has never proved possible 

 to reproduce the brilliant internal color phenomena shown by the 

 natural mineral. Opal is always cut cabochon to bring out the color 

 to the best advantage, and, while not always of great value, it be- 

 comes at times one of the most beautiful and fascinating of gems and 

 is correspondingly expensive. 



The chief commercial sources are Australia, Hungary, and Mexico. 

 Recently deposits in Nevada (pi. 9), have become important sources, 

 producing black opals of extraordinary size and beauty. 



LIST OF SPECIMENS. 

 AUSTRALIA. 



Cabochon, elliptical girdle; blue and green; 31.96 carats; 33 by 16 by 10 

 mm. Isaac Lea collection No. 1830 



New South Wales. 



Slab, rectangular girdle; mosaic; 29 by 46 mm No. 1042 



Abercrombie River: 



TVo cabochons, elliptical girdle; blue and pale yellow; 5.58 and 3 carats; 



20 by 10 by 4.5 and 14 by 10 by 4 mm. Gift of H. P. Petersen No. 1082 



Lightning Ridge: 



Six irregular fragments, polished; blue and green; greatest dimensions, 



38, 28, 25, 21, 21, and 19 mm No. 1041 



White Cliffs: 



Necklace of 50 graduated beads, separated by crystal beads; white with 



blue internal color; 9.5 to 4.5 mm. diameter No. 1083 



Cabochon, elliptical girdle; blue-green in brown matrix; 22.97 carats; 



24 by 19 by 8 mm No . 1044 



Cabochon, circular girdle; blue-green in matrix; 15.96 carats; 17 by 10 



mm No. 1047 



Cabochon, elliptical girdle; blue-green and brown in matrix; 13.94 



carats; 17 by 15 by 6 mm No. 1049 



Cabochon, elliptical girdle; green and blue in matrix; 12.48 carats; IS 



by 15 by 7 mm No. 1045 



Cabochon, circular girdle; blue-green in matrix; 12.22 carats; 15 by 6.5 



mm No. 1050 



Cabochon, elliptical girdle; green-blue and red in matrix; 11.58 carats; 



22 by 12 by 5 mm No. 1046 



Cabochon, elliptical girdle; violet-blue in matrix; 8.84 carats; 16 by 10 



by 7 mm No. 1053 



Cabochon, elliptical girdle; blue-green in brown matrix; 8.06 carats; 



18 by 15 by 4 mm No. 1048 



