Rutile
a widely distributed mineral, occurring mostly in crystals, occasionally massive; crystallization tetrag- RUTLEDGE onal; much twinned, by repetition of the same twin often assuming a geniculated appearance; hardness, 6 to 6.5; sp. gr., 4.18 to 4.25; luster, metallic-adamantine; color, red to reddish-brown, yellowishblack; streak, brown; transparent to opaque; fracture, sub-conchoidal to uneven; composition: oxygen, 39; titanium, 61-100, corresponding with the formula TiO2. Dana divides this species into: (1) ordinary, which includes the brownish-red and other shades; sp. gr., 4.18-4.22, and the acicular varieties often inclosed in rock crystal; (2) ferriferous; color black (a) nigrine, (b) ilmenorutile; (3) chromiferous, color grass-green, owing to oxide of chromium. Found distributed in granite, gneiss, mica-schists, and sometimes in granular limestones.