236 DESCRIPTIONS OP MINERALS. 



"icicles" of chalcedony, hung from the roof of the cavity. 

 Some of these chalcedony grottos are several feet in dia- 

 meter. Large geodes of this kind occur in the Keokuk 

 limestone in Iilinois and Iowa. 



Chrysoprase. Apple-green chalcedony. It is colored by 

 nickel. 



Carnelian. A bright red chalcedony, generally of a clear 

 rich tint. It is cut and polished and much used in the more 

 common jewelry. It is often cut for seals and beads. 



Sard. A deep brownish-red chalcedony, of a blood-red 

 color by transmitted light. 



Agate. A variegated chalcedony. The colors are dis- 

 tributed in clouds, spots, or concentric lines. These lines 

 take straight, circular, or zigzag forms ; and when the last 

 it is called fortification agate, so named from the resem- 

 blance to the angular outlines of a fortification. These lines 

 are the edges of layers of chalcedony, and these layers are 

 the successive deposits during the process of its formation. 

 Moclia stone or Moss agate is a brownish agate, consisting 

 of chalcedony with dendritic or moss-like delineations, of an 

 opaque yellowish-brown color. They arise from dissem- 

 inated iron oxide. All the varieties of agate are beautiful 

 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. A kind of agate having the colors arranged in 

 flat horizontal layers ; the colors are usually light clear 

 brown and an opaque white. AVhen the stone consists of 

 sard and white chalcedony in alternate layers, it is called 

 sardonyx. 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. 

 A noted ancient cameo is the Mantuan vase at Brunswick. 

 It was cut from a single stone, and has the form of a cream- 

 pot, about 7 inches high and 2|- broad. On its outside, 

 which is of a brown color, there are white and vellow groups 

 of raised figures, representing Ceres and Triptolernus in 

 search of Proserpine. 



Cat's Bye is greenish-gray translucent chalcedony, hav- 

 ing a peculiar opalescence, or glaring internal reflections, 

 like the eye of a cat, when cut with a spheroidal surface. 



