The DiaPusher plugin version 2.0 is out: with steps between coal and diamonds to make it fairer :D
http://dev.bukkit.org/bukkit-plugins/diapusher/
Hope you enjoy pressing coal to diamonds :D
PS: work for every new bukkit version(no versions are knows, where it doesn´t work)

256849003s ago, by Antonio 
I want that
256840500s ago, by AlexZhu 
Coal is made up of carbon. Diamonds are too. But diamond has had thousands of years if pressure to change it.
256844592s ago, by DaSharkCraft 
A coal to diamond
256846316s ago, by ofekDeutscher 
That real
Cool