TRANSLUCENT AND OPAQUE STONES 570 



artificially coloured apple-green chalcedony, which, to all intents and purposes, is chrysoprase. 

 Prase, plasma, and green jasper, though scarcely distinguishable when cut and polished, are 

 easily recognised in the rough, or when thin sections are examined under the microscope. 

 All three have about the same value as ornamental stones, and their distinguishing features 

 are not therefore of very great practical importance. Prase is slightly harder and denser 

 than is plasma or jasper. It is usual to apply the term jasper to stones which are perfectly 

 opaque, and plasma to those which are slightly translucent. Plasma with red spots is 

 known as heliotrope. 



Amazon-stone is of a bluish-green colour, which is never vej'y dark in shade. In hard- 

 ness it is a whole degree lower than the varieties of quartz and is easily scratched by any of 

 these. It possesses a distinct cleavage, the existence of which is very evident in rough stones, 

 and is indicated in cut specimens by the presence of numbers of straight cracks. 



Green opal (prase-opal) of the colour of prase is not very common, and is characterised 

 by its very low specific gravity. 



Glass is distinguishable from all the members of this group by its low degree of 

 hardness. 



4. BLACK STONES. 



In Division I. haematite is possessed of a metallic lustre and a red streak, while ceylonite 

 is sufficiently distinguished by its great hardness and high specific gravity. 



Black diamond is characterised by still greater hardness and by the brilliant lustre 

 possessed both by natural crystals and by cut stones. 



Obsidian and black glass are of very minor importance ; they cannot be distinguished 

 from each other by mere inspection, but require a microscopical examination of thin sections. 



Jet feels warm to the touch and can be cut with a knife ; the chai-acters in which it 

 differs from vulcanite have been given already under the description of jet. 



5. YELLOW AND BROWN STONES. 



