VARIETIES OF AGATE, ETC. 131 



having been formerly supplied by traders from Mocha in Arabia, 

 or as a corrupted form of the Latin musms through the Italian 

 vwschos — moss. Eminent mineralogists have differed in opinion 

 as to whether the markings were due to embedded vegetation or 

 to mineral infiltrations. The latter opinion now prevails, analysis 

 failing to detect vegetable substances, while various mineral 

 matters are traced in the minute cracks and porous portions of the 

 stone, giving the appearance of ferns, foliage, and moss. 



Onyx is banded agate containing bands of two or more differ- 

 ent colours. If one of the bands be red-carnelian or sard, the 

 stone is called a sardonyx. 



Brecciated Agate. — Some of these stones are very beautiful, 

 and have the various constituents — Amethyst quartz, chalcedony, 

 and jasper — being blended so as to produce contrast and harmony 

 in a pleasing manner. Specimens of these may be seen in 

 Ruskins' collections at the Sheffield Museum. It is thought by 

 some authorities that the original agate has been, perhaps, by 

 some earth movement, faulted, broken into fragments, and 

 re-cemented by infiltration of silica. 



The main constituent of agate is chalcedony, and some agates 

 are composed entirely of it ; but generally one or more of the 

 other constituents occur with it. Some agates consist of chalce- 

 dony only ; others of chalcedony and jasper ; others, again, of 

 chalcedony and quartz, chalcedony quartz and jasper, and chal- 

 cedony, amethyst, and jasper. In order to understand the com- 

 position and formation of an agate, a clear definition of these 

 various forms is almost indispensable, although difficult to give. 



Chalcedony may be defined as a crypto-crystalline form of 

 silica. According to Fuch and Dana, it is " Hydrated quartz, or 

 quartz with opal disseminated through it," and generally containing 

 a certain proportion of alumina. Bauerman says it "is essen- 

 tially a mixture of quartz and amorphous silica, forming botry- 

 oidal, reniform, or stalactitic masses, generally greyish or yellowish, 

 white, blue, rarely brown or red, and from translucent to nearly 

 opaque." It does not occur in definite crystals. It is readily 

 distinguished when it appears in a stalactitic form, but its principal 

 characteristic is its reniform structure, not only being kidney, 

 shaped, but showing under the microscope numerous incipient 



