Re: What is the difference of covalent bonding between diamond and graphite?

Diamond is sp3-hybridized carbon.  Graphite is sp2-hybridized carbon as 
planes of edge-fused aromatic rings stacked into a lamellar solid.
 http://cst-www.nrl.navy.mil/lattice/struk/a4.html 
diamond "crystal strucure" 

http://cst-www.nrl.navy.mil/lattice/struk/a9.html
graphite "crystal strucure"

For more information, see
"Advanced Inorganic Chemistry," 6th Ed., Cotton, Wilkinson, et al., 1999, 
pp. 210-212.

"Organic Chemistry," 5th Ed., Morrison and Boyd, 1987, pp. 438; 1168-9.

Note that there are several allotropes and polymorphs of elemental carbon: 
graphite, diamond(cubic)/lonesdaleite(hexagonal), fullerenes, 
cyclopolycarbyne, and SC4 (Phys. Rev. B, 53, 5051 (1996).

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