oaDEn vir. OCTAHEDRAL DIAMOND. 307 



Lustre, bright adamantine. Colour white, preva- 

 lent. Also various shades of blue, red, yel- 

 low, green, brown, grey, and even black. Ge- 

 nerally pale. Streak white. Transparent ... 

 translucent, dark coloured varieties only on their 

 edges. If cut and polished, it shews a most 

 lively play of colour. 



Hardness = 10-0. Sp. Gr. = 3-520 of a white va- 

 riety. 



Compound Varieties. Twin-crystals. 1. Face 

 of composition parallel, axis of revolution perpen- 

 dicular to a face of the octahedron. Fig. 156. 163. 

 2. Face of composition parallel, axis of revolution 

 perpendicular to a face of the hexahedron. The 

 individuals, having the general aspect of tetrahe- 

 drons, are continued beyond the face of composition. 



OBSERVATIONS. 



1. Many of the combinations occurring in the present 

 species, possess a semi-tessular character. There occur 

 also simple forms, which present the same peculiarity, as, 

 for instance, the icositetrahedron, VoL I. Fig. 25. and Fig. 

 26. Twin-crystals, like Fig. 164,* are possible only upon, 

 the supposition of semi-tessular forms. They have been 

 first described in the Catalogue of the collection of Mr 

 VON DER NULL. It will depend upon future observation, 

 whether this character is general in the present species. 



2. Octahedral Diamond is perfectly combustible at a tern- 



* This figure represents a very distinct crystal in the collec- 

 tion of Mr ALLAN. Similar varieties have been observed, 

 and models illustrative of their formation, are exhibited in the 

 British Museum. H. 



