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)

272805988s ago, by Antonio 
I want that
272797485s ago, by AlexZhu 
Coal is made up of carbon. Diamonds are too. But diamond has had thousands of years if pressure to change it.
272801577s ago, by DaSharkCraft 
A coal to diamond
272803301s ago, by ofekDeutscher 
That real
Cool