It is my belief and feeling that almost all crystals are grounding. Crystals come from the earth, and being in contact with one of them, brings me into contact with the part of the earth that they are from.
Crystals, stones, gemstones, all come from the earth. They ARE the earth. Working with them, allows the vibrations of the earth and the resonance of each crystal's particular color and frequency expression, to enter your field.
For the purposes of this share, I'm just talking about using crystals for grounding. There are so many other ways to work with your crystals, too!
1) Black Tourmaline; I have a big old chunk of this on the dash in my car. I grab it and palm it when needed. I also buried pieces of black tourmaline in the four corners of my yard, to protect my home. All black crystals are considered to be grounding. They absorb negativity, and root you to the earth.