It is most important to cleanse your crystals before you wear/use them. This is not just washing them in soap and water to get rid of the dirt!
They take on energy and store this energy. So if the energy they have taken on before you get them, is negative, then this energy will be there when you wear or use the crystal. Cleaning a crystal leaves a blank slate that will become personal to you.
There are a number of different opinions on the way to cleanse crystals from leaving them in running water (natural spring) for a month, to cleansing them by fire.
The first of these is really not practical for most of us because of time and the proximity of an appropriate natural spring, and the fire option is a bit dramatic!
Some stones will be adversely affected by fire. The following are two more convenient and also very effective ways of cleansing:
Salt can be very harmful to some stones, for example, an opal left in salt will be ruined. So, to ensure there is no damage to your stone, but to ensure an effective and quick cleansing with salt, place the crystals in a glass dish which is in turn embedded in to a larger dish which is full of salt. The cleansing energy is maintained but the stone is safe.
Salt can be re-used for months, so it is also a cost effective and non-wasteful way to cleanse the crystals. If the stone has been worn for some time, i.e. for weeks, then 4 -6 hours will be enough. If the crystal has only been laid on briefly, then 10 minutes should suffice.
Another way to cleanse your crystal is in water. Pure water with a few drops of Rescue Remedy (Bach Flower Remedy) in the water. 24 hours is sufficient to cleanse your crystal.