Some see the cleansing of a crystal as returning it to its natural consciousness as it was in the earth.
Before a crystal reaches us they go through certain kinds of trauma which can attract negativity …... when they are extracted from the earth, when they were cleaned and polished, when they are handled by several different people - who knows what those people were feeling at the time and the energy they may have passed into the crystals, and many crystals would have travelled half way round the world to get to us.
So to be sure that a crystal is free of all this negative energy it is important that they are cleansed.
I believe that the ritual of cleansing the crystal allows you to get to know it and for your energies to merge and tune into one another. The crystal responds to your intention to cleanse it, and what ever method you use just makes that intention stronger.
When crystals and stones are being used for healing or therapy it is vital that they are cleansed after working with each person. The crystals can pick up and keep the energy of that person and you do not want their energy interfering with the healing of anyone after them.
For the crystals which are around the home or not used for healing they may not need as much cleansing. You can also tell when a crystal needs cleansing for they become 'sticky' and dull ( their shine may be less brilliant ) and clouded.
To summarise :
A. Perform an initial cleansing when you first get the crystal
B. Perform regular cleansing of your crystals