20 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



CHALCEDONY. 



Varieties. — Here are included the cryptocrystalline and amor- 

 phous, often more or less impure, varieties of silica comprised under 

 the names agate, bloodstone, carnelian, chrysoprase, jasper, prase, etc. 

 The term agate includes the banded forms (fig. 2); carnelian, the red; 

 chrysoprase, the green; bloodstone, a compact, dark, opaque variety 

 with blood red spots; jasper, a great range of opaque impure forms 

 of a red, yellow brown, or black color, the varying hues being due 

 mainly to iron and manganese oxides. The silicified wood of the 

 Arizona " Fossil Forest" is largely chalcedony in the cryptocrys- 

 talline form of jasper; onyx is a variety of agate with straight alter- 



Fig. 2.— Banded agate. 



nating bands of light and dark; moss agate, a milky or colorless form 

 with dendritic markings of manganese oxide resembling moss or 

 other plant growth. Plasma and prase are green in color, and sard, 

 or sardonyx, of a golden to blood red color. 



These stones are cut only cabochon or flat, unfacetted forms, and 

 are valued according to their varying beauty, common agate and 

 jasper being the cheaper forms. The majority of agates sold in the 

 shops are from Brazil, and are cut and artificially colored in Germany. 



The moss agates of Wyoming are found, according to Mr. C. J. 

 Hares, of the United States Geological Survey, scattered over the 

 surface of the ground in several townships of Fremont County and 

 along the Sweetwater Valley. The agate pebbles range in size from 

 2 inches or more in diameter and are usually well rounded. The 

 good specimens are uncommon, being associated with a great many 



