QUARTZ (CAT'S-EYE) 491 



possible to the surface which displays the rainbow colours, this being not plane but more or 

 less irregular and curved, corresponding to the conchoidal fracture of quartz. Rainbow- 

 quartz is worth more than rock-crystal, and specimens which show a central portion 

 with a play of colours surrounded by a colourless border have a considerable value. In 

 most cases the iridescence is confined to certain portions of the mass of rock-crystal, and 

 these must be carefully sawn out in order to be cut for ornamental stones. Large crystals 

 are occasionally iridescent throughout their whole mass and can be used in the fashioning 

 of articles of large size, such, for example, as the candelabra in the Vatican collection, which, 

 however, may possibly be made up of several portions. 



More or less fine specimens of rainbow-quartz are found occasionally at all the localities 

 at which ordinary rock-crystal occiu's. Iridescence can be produced in the latter by 

 striking it with a hammer or by immersing it in cold water after it has been strongly heated, 

 the object in both cases being to develop cracks in its substance. Specimens are, of course, 

 often broken by subjection to this treatment, but this is of little consequence considering 

 the abundance of the mineral. Many other transparent minerals display iridescent colours, 

 and ai-e cut for ornamental stones, all of which are known by the same name of " iris." 

 This term is also applied to cut rock-crystal and also to paste, made to imitate an 

 iridescent stone by the use of vari-coloured foils, and much used in the cheapest of 

 jewellery. 



CAT'S-EYE. 



The term cafs-eye (quartz-cafs-eye, occidental cafs-eye, schiller quartz) is applied 

 to massive quartz from the surface of which, especially when cut in a rounded form, 

 there is reflected a wave of milky light. The appearance is exactly the same as that 

 of cymophane, a variety of chrysoberyl, also known as cafs-eye, but distinguished from 

 quartz-cafs-eye by the pi-efix oriental. Unfortunately the distinction between occidental 

 and oriental cafs-eye is not always made, an omission which leads to much confusion, 

 especially in statements respecting the occurrences of these stones. They differ very 

 markedly in almost every character ; thus cymophane or oriental cafs-eye is far more 

 brilliant and beautiful and much less common than is quartz-cafs-eye; and the band 

 of chatoyant light is more clearly defined in the former than in the latter. In coiTCspondence 

 with these differences is the fact that cymophane is a much more valuable stone than 

 is occidental cafs eye. The two stones may be easily distinguished by their difference 

 in hardness, chrysoberyl being harder than topaz, and therefore much harder than quartz, 

 and in specific gravity. Chrysoberyl has a specific gravity of 3'7 and therefore sinks 

 heavily in methylene iodide, while quartz with a specific gravity of 2'65 floats in that 

 liquid. 



Quartz showing the optical effect known as chatoyancy occurs in compact masses, 

 which are not, however, aggregates of single grains, but of uniform crystalline structure 

 throughout. It has a somewhat greasy lustre, is never transparent, but always more or 

 less translucent. The colour is sometimes white, but more frequently olive-green to 

 dark leek-green, these paler or darker shades of green always containing grey tones. 

 The mineral also occurs of a pronounced brown or yellow colour and red tinged with 

 various shades of brown and yellow. Blue stones occur as rarities. Green and yellowish- 

 brown stones cut en cabochon are represented in Plate XVIII., Figs. 4a and b. 



The chatoyancy of quartz-cafs-eye is due to the presence of large numbers of 

 fibres of asbestos, which are embedded in perfectly parallel directions in its substance 



