TERMINOLOGY. . 203. 



The Play of Colours is produced, if the mineral in certain 

 directions reflects as it were coloured points of great inten- 

 sity, which change with the position of the mineral, or with 

 the direction of the rays of light. Of this property, octa- 

 hedral Diamond, if cut, and precious Opal, a variety of un- 

 cleavable Quartz, both cut and in its natural state, are 

 quoted as examples. The play of colours in octahedral 

 Diamond depends upon the reflexion of refracted light, 

 occasioned by the artificial facets ; in precious Opal it is 

 more analogous to the change of colours and the opalescence. 



The Change of Colours consists in the reflection of bright 

 hues of colour, in certain directions depending upon the 

 structure of the mineral. The mineral which presents the 

 change of colour in the most remarkable degree, is La- 

 bradore felspar, a species of the genus Feld-spar. 



The Opalescence consists in a kind of milky light, which 

 certain minerals reflect, either if cut en cabocfion, or upon 

 plane faces both natural and artificial. It is, like the preced- 

 ing property, analogous to the play of colours in uncleavable 

 Quartz. In the varieties of rhombohedral Quartz, called 

 Cats eye, it depends upon composition ; in prismatic Co- 

 rundum, and in the transparent varieties of prismatic Feld- 

 spar, called Moonstone, it depends upon the crystalline 

 structure. Upon this structure it likewise depends in 

 rhombohedral Corundum, and in dodecahedral Garnet : 

 this appears in particular in the six-sided and four-sided 

 stars of light, from which the varieties of the former have 

 received the name of Asteria. 



The Iridescence shews the colours of the rainbow, simi- 

 lar to those produced by the refraction of light, through a 

 prisjn of glass. It presupposes fissures or separations in 

 the interior of the minerals, which may depend on struc- 

 ture or on composition, or which may even be entirely ac- 

 cidental. The included space, not filled up by the mineral, 

 shews the phenomenon of the coloured rings, sometimes 

 very bright as in rock-crystal, a variety of rhombohedral 

 Quartz, where it is occasioned by accidental fissures in the 

 interior. Another variety of the same species, called the 



