Here is one thing to remember, EVERYTHING has polarity that include crystals. That which can cure can kill. As a crystal programmed for cleansing does its thing depending on how itnis doing it, energy will either be repeled by the crystal or attacted into it to cleanse the area and over time polarise into its negative state.
This is why it is a good idea to cleanse a dn charge all crystals periodically even those that specialize in cleansing. The vibrational strength of that charge is not infinite, even though the frequency ofbthe crystal specializes in one thing or another whether it is cleansing negative and toxic energies or transmuting that energy into positive like obsidian.