ORDER VII. UNCLE AVABLE QUARTZ. 353 



grey ; none of them lively, except some red and 

 green ones, and generally pale; dark colours 3 

 owing to foreign admixtures. Streak white. 

 Transparent ... translucent, sometimes only on 

 the edges, or even opake, if the colours be very 

 dark. Lively play of light observable in some 

 varieties ; others shew different colours by re- 

 flected and refracted light. 



Hardness = 5-5 ... 6-5. Sp. Gr. := 2-091, a milk- 

 white variety ; = 2-060 a brownish-red variety. 



Compound Varieties. Small reniform, botryoi- 

 dal, and stalactitic shapes, and large tuberose con- 

 cretions : surface of the former smooth, of the lat- 

 ter rough ; composition impalpable, fracture con- 

 choidal. Massive, composition impalpable; fracture 

 conchoidal, even. Pseudomorphoses of rhombohe- 

 dral Linie-haloide. 



OBSERVATIONS. 



1. The phenomenon of the play of colour of precious 

 Opal has not been hitherto satisfactorily explained. Ac- 

 cording to' HAUY, it is the consequence of fissures in the 

 interior filled with thin films of air, which reflect coloured 

 light according to the law of NEWTON'S coloured rings. If 

 this were the fact, Opal would present nothing else but a 

 kind of iridescence, and the beauty of Opal would be owing, 

 as HAUY expresses it, only to its imperfections. But. these 

 colours often keep constant directions within single parts 

 of the mass ; and in specimens not cut in the usual convex 

 form, but presenting even faces, it is often possible to ob. 

 serve distinct images reflected, exactly as in the Moonstone, 

 which is a crystallised variety of prismatic Feld-spar, or in 

 prismatic Corundum. The play of colour seems therefore 



