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)

188919738s ago, by Antonio 
I want that
188911235s ago, by AlexZhu 
Coal is made up of carbon. Diamonds are too. But diamond has had thousands of years if pressure to change it.
188915327s ago, by DaSharkCraft 
A coal to diamond
188917051s ago, by ofekDeutscher 
That real
Cool