Just by being mindful enough to ask this question you move yourself, and what you are doing, into the “safe zone.” You cannot do harm via crystals. The “harm” (and this is true in all situations) can come from the personal dynamics that become injected into a situation – like wanting to prove that crystals work. The initial techniques, where you create a set up for crystals to emit their healing energy into the energy field, have built in “safety” mechanisms. Like putting a sponge in a bucket of water, once the sponge is saturated, it simply no longer absorbs more. The more advanced Crystal Surgery techniques, where you are actually “operating” on the person do require a mindfulness to work only one layer at a time. Precautions, where necessary, are clearly included in the instructions. Having said that, the worst that can happen, is that you can provide an “overdose” in an attempt to “feel” the crystal energy. An overdose can render the client “too sensitive” or “too spacey.” Such an effect will dissipate with time, leaving no harmful after effect.