This is something I wrote in the board a few months ago in the Dream Section:
The stone type can change the method you use to recharge. Some stones are self cleaning, others can cleanses stones.
How can do the following to cleanse a crystal:
*hold under running water
*put the stone in salt water(can also be the ocean) and recharge it by rejecting and negative energy the stone may be holding
*place the stone in sunlight or moonlight for a few hours
* i think I have read you can use fire, but very quickly.
And to add on, you can also place stones in the earth and bury over night. And just like theprototype said, you should put your intentions onto the stone.