12 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



bility. It occurs in veins and pockets and has been derived from 

 sulphide ores through decomposition by surface water, as has mala- 

 chite, with which it is very commonly associated. Used for orna- 

 ments, scarfpins, etc. ; the value as a gem is little more than the cost 

 of cutting. (See under Malachite.) 



BENITOITE. 



Composition. — Barium titano-silicate, BaTiSi 3 9 . 



Crystallization. — Hexagonal (trigonal), holohedral. 



Color. — Pale to deep blue; pleochroic. 



Luster. — Vitreous. 



Hardness. — 6.5. 



Specific gravity. — 3.64 — 3.67. 



Optical properties. — Refractive index 1.757 to 1.804. Double re- 

 fraction strong and positive. High index of refraction and marked 

 pleochroism are distinguishing features of the mineral. So far as at 

 present known it occurs only in natrolite veins cutting serpentine. It 

 is usually cut as brilliant, more rarely cabochon. A limited supply 

 of this mineral has as yet been found (pi. 4). It is, therefore, little 

 used, though of good quality. 



LIST OF SPECIMENS. 



UNITED STATES. 



California. 

 San Benito County: 



Two gems, brilliant, circular girdle; blue and violet blue; 0.95 and 0.87 carats, 

 5.7 by 4.5 and 5.5 by 4 mm. Isaac Lea collection No. 1180. 



BERYL. 



Varieties. — Common, aquamarine, emerald, morganite (vorobie- 

 vite), and golden beryl. 



Composition. — Beryllium aluminum metasilicate, Be 3 Al 2 (Si0 3 ) 6 . 



Crystallization. — Hexagonal; habit usually prismatic, rarely 

 tabular. 



Color. — Colorless when pure; often showing green, blue, yellow, or 

 pink hues, and in part named accordingly: Pale blue, green-blue, 

 or blue-green, aquamarine; intense green, emerald; yellow, orange- 

 yellow, or brown-yellow, golden-beryl; and pink, morganite, or voro- 

 bievite. Slightly pleochroic, if deeply colored. 



Luster. — Vitreous ; transparent. 



Hardness. — 8 (less when altered) ; a durable stone. 



Specific gravity. — 2. 75 ±0.10. 



Optical properties. — Mean refractive index 1.58; double refrac- 

 tion weak, 0.005; uniaxial, negative. With the microspectroscope 

 emerald yields two absorption bands in the orange, which form a 

 characteristic distinction from green tourmaline, green corundum, 



