136 SILICA. 



tiful stones when polished, but are not much used in fine 

 jewelry. The colors may be darkened by boiling the stone 

 in oil, and then dropping it into sulphuric acid. A little oil 

 is absorbed by some of the layers, which becomes blackened 

 or charred by the acid. 



Onyx. This is a kind of agate with the colors arranged 

 in flat horizontal layers. They are usually light clear brown 

 and an opaque white. When the stone consists of sard 

 and white chalcedony in alternate layers, it is called sar- 

 donyx. 



Onyx is the material used for cameos, and is well fitted 

 for this kind of miniature sculpture. The figure is carved 

 out of one layer and stands in relief on another. The most 

 noted of the ancient cameos is the Mantuan vase at Bruns- 

 wick. It was cut from a single stone, and has the form of a 

 creampot, about 7 inches high and 2£ broad. On its out- 

 side, which is of a brown color, there are white and yellow 

 groups of raised figures, representing Ceres and Triptolemus 

 in search of Proserpine. The Museo Borbonico contains an 

 onyx measuring eleven inches by nine, representing the 

 apotheosis of Augustus ; and another exhibiting the apothe- 

 osis of Ptolemy on one side and the head of Medusa on the 

 other. Both are splendid specimens of the art, and the 

 former is supposed to be the largest in existence. 



Cat's eye. This is a greenish-gray translucent chalcedo- 

 ny, having a peculiar opalescence, or glaring internal reflec- 

 tions, like the eye of a cat, when cut with a spheroidal sur- 

 face. 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, Hornstone. Flint is massive compact silica, of dark 

 shades of smoky gray, brown, or even black, and feebly trans- 

 lucent. It breaks with sharp cutting edges and a conchoid- 

 al surface. It is well known as the material of gun-flints. 

 It occurs in nodules in chalk : not unfrequently the nodules 

 are in part chalcedonic. Hornstone resembles flint, but is 

 more brittle, and therefore unfit for making into flints. It is 

 found in limestone, and one of these rocks is called cherty 

 limestone, from the abundance of it. 



Plasma. This is a faintly translucent variety of chalce- 



How may the colors of agate be deepened ? What is onyx? For 

 what is it used 1 What are some of the remarkable camecs ? What 

 is cat's eye ? What is flint ? How does it differ from hornstone. 



