Cleansing a crystal is a useful and important thing to do. Unless a crystal comes from a special place, typically you do not want the energies it may be carrying with it. To remove energies attached to the crystal and bring it back to its neutral state, the crystal must be cleansed. Selenite has the ability to both cleanse a crystal and charge it, very rapidly at that. Placing a crystal on selenite or near it, preferably touching it, will let selenite do its work. Kyanite is another crystal that is extremely good at cleansing, it will neutralize the energy in a crystal touching it, and can cleanse crystals many times larger than it.
Charging a crystal is very useful to get more effects out of your crystals when you wear, keep, or place them around you. Crystals can store a finite amount of energy to supercharge their effects, once the stored energy is released it will go back to releasing its natural amount of energy until it is then charged again. Place a crystal on top of a charging crystal to charge it and if the charging crystal has a point then point the crystal towards the crystal being charged. Selenite is powerful at charging a crystal. Quartz is very good for this purpose as well as are all varieties of quartz, especially Citrineand Amethyst. A cluster will typically charge even better than a single crystal will as the numerous points speed up the process quite a bit. White crystals in general will have charging abilities.
Selenite is a must have for both cleansing and charging crystals.