the leads of your pencils and the
diamond is only carbon... But
physically, diamond and graphite
are different . Indeed diamond and
graphite are identical by their
chemistry, but physically different
because their carbon atoms
are not arranged in the same
manner.

This difference of arrangement is responsible for meaningful physical differences between graphite and diamond. For example we can say that graphite is very soft (the lead is easy to break) and opaque to light. On the other hand the diamond is very hard and transparent to light. The structural differences between these two species is explained below.

Graphite is hexagonal crystal. It means that groups of carbons are separated and don't form a strong block. They can slip each other as the book sheet.

The groups of carbon diamonds are bound solidly each other, there are not free groups of carbon atoms.

The diamond can turn into graphite...

 ...but to transform your pencil lead to diamond diamond is very difficult but not impossible.
 
The spontaneous transformation of diamond in graphite take place at 1500°C. In other words a match is sufficient to transform your diamond in pencil lead. Unfortunately to transform graphite in diamond is a lot more complicated.

The synthetic diamonds

They are manufactured since 1955 according to a process requiring a pressure of 55000 bars and a minimal temperature of 2000°C. The synthetic diamonds are all intended to industrial use.