This is a technique I use often to cleanse my crystals and gemstones of any negative energy. I would like to add to this that if using moonlight to cleanse your crystals or gemstones it would be even better if you can time it so that the full moon's rays cleanse the stones.
Cleansing Crystals with Sunlight and Moonlight
The easiest of the cleansing techniques involves using your stone in a windowsill where the direct sunlight or moonlight will hit it. The number of days or nights you'll have it will depend on how much negative energy has collected in the stone. Set the stone on a mirror to enhance the effect. Be careful not to handle that mirror so that is flashed the reflection of the light into the eyes, as it can damage eyesight,. Make sure that the reflection doesn't focus on a flammable surface for an extended time, either.