94 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



Alters to black manganese oxides in moist air. The mineral is 

 distinguished by its color, rather high specific gravity, and optical 

 properties. It is sometimes cut cabochon and used for scarf pins, 

 buttons, etc., but more often carved into ornaments. Worth little 

 more than the cost of cutting. 



LIST OF SPECIMENS. 

 RUSSIA (URAL MOUNTAINS, EKATERINBURG DISTRICT). 



Necklace of 47 beads; rose cut; dull red; 10 mm. diameter No. 381 



Two sleeve buttons, circular; dull red; 35 mm. diameter No. 383 



UNITED STATES. 



California. 

 Happy Camp, Siskiyou County: 



Cabochon, circular girdle; light red and gray; 10.715 carats; 18 by 4 mm.. No. 370 



Massachusetts. 

 Cummington, Hampshire County: 



Flat ellipsoid; dull pink; 36 by 26 by 12 mm No. 384 



New Jersey. 



Trotter Mine, Franklin, Sussex County: 



Cabochon, elliptical girdle; pink; 53 by 40 by 9 mm No. 382 



Rock crystal. — See under Quartz. 

 Ruby. — See under Corundum. 



RUTILE. 



Variety. — Nigrine . 



Composition. — Titanium dioxide, Ti0 2 ; may also contain small 

 amounts of iron, vanadium, chromium, and other metal oxides. 



Crystallization. — Tetragonal; habit usually prismatic, and at times 

 acicular (needlelike) . 



Color. — Usually red, varying to black in the variety nigrine, these 

 colors being largely due to the impurities, especially vanadium 

 oxide; pleochroism distinct. 



Luster. — Adamantine to submetallic; transparent to opaque. 



Hardness. — 6.5; fairly durable. 



Specific gravity. — 4.20±0.10. 



Optical properties. — Mean refractive index 2.75; double refraction 

 extremely high, 0.3; optically uniaxial, positive. 



The mineral may as a rule be identified by the high specific gravity, 

 refractive index, and double refraction. Occurs in metamorphic 

 rocks of various kinds and often in acicular crystals penetrating 

 quartz, giving rise to the forms known as sagenitic quartz, Venus's 

 hair stone, fleche d'amour, etc. Rutile is sometimes cut facetted, 

 but the color is too dark and not very attractive, hence it is not widely 

 used as a precious stone. Sagenitic quartz often yields ornamental 

 stones. Rutile is worth little more than the cost of cutting. 



