COLLECTION OF MINERALS. 290 



Wi th the twentieth case commences the series 

 of specimens which present quartz or rock- 

 crystal in its various modifications. They are 

 generally called agates ; the greater number is 

 concrete bodies, some only offering the primitive 

 form of quartz. Amongst them we may distin- 

 guish chalcedony which is of a milky white, 

 with a cloudy transparency; cornelian, of a 

 cherry red \ sapphirine, of a delicate blue ; sardo- 

 njXy of an orange colour ; prase and plasma. 

 which present different shades of green ; other 

 varieties present coloured zones, sometimes pa- 

 rallel and sometimes circular, they are then called 

 ribband and onyx agates. Artists cut them, as 

 well as the preceding, into boxes, vases, seals, 

 and other ornaments. In the lower part of the 

 case is a simple onyx agate in chalcedony, from 

 the environs- of Oberstein. Two varieties, far 

 less inviting from their appearance ,. are valuable 

 for their use, as the gunflint and the millstone. 



After the agates, come the quartz-resinitc, 

 which shines like rosin ; and the jaspers, which are 

 opaque, the fractures of which are always dull. 

 Amongst the former we observe the hydrophane y 

 which becomes transparent when plunged into 

 water ; the girasol, which reflects red and 

 gold yellow colours; the common opal, which 

 is so much sought after for the variety and beauty 



