Why are some properties of diamond different from those of graphite?
Diamonds have a tetrahedral-like composition while graphite has a hexagonal-like arrangement layered in sheets.
Diamonds and graphite are both allotropes of Carbon.
Carbon is a very common example because of how identifiable it is, most commonly diamonds and graphite.
Diamonds form when the carbon atoms are bonded together in a tetrahedral-like lattice arrangement. This is the result of high temperature and pressure at the same time over a long period of time, deep in the Earth's mantle.
It's because the carbon atoms are rearranged in a specific geometric shape that gives the diamond its properties. This results in a diamond's hardness, extraordinary strength and durability. In addition, because of its tetrahedral-like structure, diamonds are practically resistant to compression.
Graphite however, is when the carbon atoms bond together in sheets - hexagonal-like lattice. Graphite is formed from the carbon atoms resulting from the metamorphism of minerals such as marble, quartz, etc. The arrangement of the molecules are a result of a different physical structure.
Graphite's structure of sheets are soft because of its intermolecular forces. Being incredibly weak, the graphite's properties (mainly softness) are a result of the strength of the force.
Hope this helps :)