A crystal can be programmed negatively as well as positively. Bearing in mind all rocks and metals have healing properties, and that there is no such thing as a "bad" rock, I would suggest that one cleanses all new stones that they acquire before using them for healing.

Cleansing removes unwanted energy and restores them to clarity - cleansing each time before a healing or during illness is important. If you wear the stones a weekly or if it is only decretive a monthly cleansing is needed.

• Use tepid water with a mild detergent, holding stones under running water for one minute
  
• Visualizing them being cleansed of all negativity and restoring them to a positive energy.
  
• Submerge crystals in dry brown rice for 4 hours
  
• Vibrations of sound - bell, gong, singing bowl.
  
• Smudging - holding crystals in the smoke of Sage or Cedar. (Use a feather to fan smoke across the crystals)
  
• Crystal cleansing spray - spray atomizer is an easy and covenant way to clear away all negative energy on a daily basis, during illness or after an argument.
  
• Salt - be careful check if method is not going to harm the crystal especially if pores.