Black tourmaline is one of my favorite stones for protection and dissolving negative energy. I keep one at my doorway to keep the energy of the house clear at all times.
This stone is also a stone that brings stability and grounding to the physical body. When you are grounded and fully present, you are more able to create boundaries for yourself. The black tourmaline can assist with this.
If you are experiencing negative surroundings or presences in your life this is the stone for you.