Stones that are used for healing and magic need to be cleansed and charged regularly. I think it's best to cleanse and charge a stone after any healing work; stones I use as part of my own ritual circles or for meditation can go a little longer, provided my environment is reasonably harmonious and clear.
There are many ways 
to cleanse and charge your stones. Purification is often done with incense or smudge stick smoke, or with water and salt. The cleansed stones are then often charged with moonlight, sunlight, reiki, or a witch's intentions. I frequently use some combination of these methods myself.
