The structures of diamond and graphite explain these differences. The carbon atoms in diamond are each bound to 4 others in a 3-dimensional network. To change the shape of a diamond or break it requires that many of these strong bonds be broken simutaneously. Since there is little room for other molecules to get into the structure, diamond is not very reactive. Click on the image below to view the structure of diamond.
Graphite consists of sheets of strongly bonded hexagonal rings.
Each sheet is far from and weakly bound to the next. Thus, the sheets can slide past one another, making graphite a soft lubricant. The spaces between the layers allow other molecules to enter, explaining the absorbing and catalytic properties. Click on the image below to view the structure of graphite. The structures of graphite and diamond have long been known.