Amorphous forms   
        
       Different amorphous allotropes of carbon are: Coal; Coke; Wood
   Charcoal;Animal charcoal;Lamp black;Gas carbon; Petroleum coke;Sugar
   charcoal.   
            
   Crystalline forms     
            
       Carbon atoms can arrange themselves into different hybridised
   chemical bonds. Therefore they exhibit different physical and chemical
   structures such as diamond and graphite.Carbon in solid phase can exist
   in three crystalline allotropic forms : diamond, graphite and
   buckminsterfullerene.    
            
       Diamond and graphite form covalent network structures whereas
   buckminsterfullerene has a molecular solid structure with discrete C60
   molecules.As these crystalline allotropes differ in their structures,they
   possess different physical properties. 
   
   Diamond
      
       In diamond each carbon atom undergoes in its excited state sp3
   hybridisation.Hence, each carbon atom has a tetrahedral environment.The
   three dimensional structure of diamond is as shown:      
           
     
       As C-C bonds are very strong any attempt to distort the diamond
   structure requires large amount of energy.Hence diamond is one of the
   hardest materials known.
    
   Graphite
   
      Graphite forms a two dimensional layer structure with C-C bonds within
   the layers. There are relatively weak interactions between the layers.
   
      In the layer structure, the carbon atoms are in a trigonalplanar
   environment.This is consistent with each carbon atom in sp2 hybridisation.
   Interactions between the sp2orbitals (overlaps) lead to the formation of
   C-C bonds. Each carbon atom is with one unhybridised ‘p’ orbital. The