Black Tourmaline is electrical in nature and provides a connection between the Earth and base chakra (which is the energy centre that connects you to the Earth). This is why it is great to use for grounding and protecting your energy field. It will create an equivalent of an electrical force field around your aura so lower vibrations will bounce straight off you.
You can use this stone to purify and neutralize your own thoughts and internal conflict, it will even transform dense energy into a lighter vibration. You can also use this to help protect yourself against electromagnetic smog from mobile phones and computers etc. Although I would highly recommend looking into Shungite stone if you are looking for the best protection against EMF.
Black Tourmaline promote detoxification, relieve stress and anxiety, increases alertness and protects you from stressful and negative environments. It really works on all areas physically, emotionally and mentally.
I use it regularly to help ground and protect myself, clients and sensitive kids from any lower harmful energies. They can put in in their pocket or carry it in their school bag. I even place it in their pillow slip if need be.
This stone is also great for protecting a space by placing a stone in each corner of the room creating a crystal grid of protection.
Be sure to regularly cleanse your crystals. https://www.adelaidereiki.com.au/cleansing-and-charging-your-crystals/