236 DESCRIPTIONS OF MINERALS. 
‘icicles ” of chalcedony, hung from the roof cf the cavity. 
Some of these chalcedony grottos are several feet in dia- 
meter. Large geodes of this kind occur in the Keokuk 
limestone in Lilinois and Lowa. 
Cirysoprase. Apple-green chalcedony. It is colored by 
nickei. 
Carnelian. <A bright red chalcedony, generally of a clear 
rich tint. Itis 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 Guring the process of its formation. 
Mocha 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 Jayers; the colors are usually ght clear 
brown and an opaque white. When the stone consists of 
sard and white chalcedony in alternate layers, it is called 
sardonyx. Onyx is the material used for cameos, and 1s 
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 23 broad. On its outside, 
which is of a brown color, there are white and yellow groups 
of raised figures, representing Ceres and Triptolemus in 
search of Proserpine. 
Cat’s Eye is grecnish-gray translucent chalcedony, hav- 
ing a peculiar opalescence, or glaring internal reflections, 
like the eye of a cat, when cut with a spheroidal surface. 
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