v 

 OKDEK VII. IUIOMBOHEDRAL QUARTZ. 323 



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3 



P 4. QD. [( p +g)!]. Fig. 147* Chamouny. 



Irregular forms, grains. 



Cleavage, P and P + QD. The alternating faces 

 of the pyramid, corresponding to P, are more 

 easily obtained than the rest, but all of them are 

 imperfect, and interrupted by conchoidal frac- 

 ture. Fracture conchoidal, sometimes highly- 

 perfect, sometimes less distinct.-)- Surface, P -j- 1, 



1 1 



/p\ 5 fP\^ 



sometimes also P 4- 2 and !LZ are rough, 1 ' . 



2 2 



is streaked parallel to the edges of combination 

 with R, R sometimes parallel to the same line. 

 P + oo horizontally streaked, sometimes parallel 

 to the edges of combination with R. Thfe rest of 

 the faces generally smooth. 



Lustre vitreous, inclining in some varieties to resi- 

 nous. Colour, white prevalent; among the 

 brightest colours are violet-blue, rose-red, clove- 

 brown, and apple-green. Dark brown and green 

 colours generally owing to foreign admixtures. 



A splendid crystal of this variety, of considerable size, is 

 in the cabinet of Mr ALLAN. H. 



f The fracture perpendicular to the axis, often possesses the 7 

 appearance of velvet, particularly in transparent varieties, aj' 

 has been first observed by Dr BREWSTER. 



