The normal diamond structure is cubic the hexagonal form has been found in metorites.
Cubic Diamond: Open the diamond.ins file using Oscail it should be in \ortex.
Click 
to start Ortex, accept defaults and click 
to go to atom mode. Click 
and select Lattice Pack with Limits and OK. On the Lattice Pack
Dialog set the limits as shown and click OK.
Examine the structure carefully and you should be convinced that there are cyclohexane chairs in the horizontal and vertical directions.
Hexagonal Diamond: Repeat this procedure with hexagonal diamond. The file is diamondhx.ins. Use the pack range 0-2.5, 0-2.5, 0-2.0.
Examine this carefully and the cyclohexane charis are again visible in the horizontal (ab) direction while there are boats in the vertical direction.