166 PHYSIOGRAPHY, 



Diamond. 



DIAMOND. Octahedral Diamond. MOHS. 

 Primary form. Regular octahedron. 

 Secondary forms. 



1. Primary form, having its solid angles replaced by tan- 

 gent planes. 



2. Primary form, having its edges replaced by tangent 

 planes. 



3. Cube. 



4. Cube, having its edges replaced by tangent planes. 



5. Rhombic dodecahedron. 



The annexed figure repre- Fi g- 158 - 



sents in planes P P' P" and 

 P'" the primary faces of the 

 octahedron, and in a a' a" 

 those of the cube, which are 

 generally flat and brilliant. 

 The numerous faces di and 

 d2, are uniformly convex; 

 each of which is, in reality, a 

 series of planes, as is mani- 

 fest on other crystals, but in no instance sufficiently perfect 

 for the use of the reflective goniometer. 



Irregular forms and grains. 



Cleavage, parallel with the primary octahedron, perfect. 

 Fracture, conchoidal. Surface, the octahedron sometimes 

 faintly streaked parallel to its edges of combination, but in 

 general very smooth. Also the dodecahedron, if often 

 streaked, rough and uneven ; the tetraconta-octahedron, 

 curved and smooth. Grains possess a rough and granula- 

 ted surface. 



Lustre bright adamantine. Color white, prevalent ; al- 

 so various shades of blue, red, yellow, green, brown, grey, 



