silica. 237 



The effect is owing to filaments of asbestus. It comes from 

 Ceylon and Malabar, ready cut and polished, and is a gem 

 of considerable value. 



Flint, Homstone. Massive compact silica, of dark shades 

 of smoky gray, brown, or even black, and feebly translucent, 

 it breaking with sharp cutting edges and a conchoidal sur- 

 face. Flint occurs in nodules of chalk: notunfrequently the 

 nodules are in part chalcedonic. Hornstone differs from 

 flint in being more brittle ; it is of ten found in limestone. 



Chert is an impure hornstone. Limestones containing 

 hornstone or chert are often called cherty limestone. 



Plasma. A faintly translucent variety of chalcedony ap- 

 proaching jasper, of a green color, sprinkled with yellow 

 and whitish dots. 



in. JASPERY VARIETIES. 



Jasper. A dull red or yellow siliceous rock, containing 

 some clay and yellow or red iron oxide, the red, the anhy~ 

 drous oxide, and the yellow, the hydrous oxide. Heat drives 

 off the water from the yelloAV jasper and turns it red. It 

 also occurs of green and other shades. Riband jasper is a 

 jasper consisting of broad stripes of green, yellow, gray, 

 red, or brown. Egyptian jasper consists of these colors in 

 irregular concentric zones, and occurs in nodules, which 

 are often cut across and polished. Ruin jasper is a variety 

 with delineations like ruins, of some brownish or yellowish 

 shade on a darker ground. Porcelain jasper is nothing but 

 a baked clay, and differs from jasper in being fusible before 

 the blowpipe. Red felsyte resembles red jasper ; but this 

 is also fusible, and consists largely of feldspar. 



Jasper admits of a high polish, and is a handsome stone 

 for inlaid work, but is not much used as a gem. 



Bloodstone or Heliotrope. Deep green, slightly trans- 

 lucent, containing spots of red, which have some resem- 

 blance to drops of blood. It contains a few per cent, of 

 clay and iron oxide mechanically combined with the silica. 

 The red spots are colored with iron. There is a bust of 

 Christ in the royal collection at Paris, cut in this stone, in 

 which the red spots are so managed as to represent drops 

 of blood. 



Lydian Stone, Touchstone, Basanite. Velvet-black and 

 opaque, and used, on account of its hardness and black 

 color, for trying the purity of the precious metals ; this is 



