It you are looking for a blocking or grounding stone, I would recommend Red Jasper, it is known as the Warrior Stone, Black Obsidian or Hematite.
I would suggest starting out with a small stone so that you can familiarize/bond yourself with the stone.
If you have a shop near you I would suggest going in person so that you can see the stones and see which stone us a best fit for you.
Some stones may give you a headache or just may not feel right to you and some just may have that pull or draw that makes you keep hovering over it.
I also cleanse my stones by either placing them within the smoke of incense or under the full moon to get rid of the built up energy.
I then take some time and hold each stone in my hand and focus with intention on what I want or need for that stone to do for me.
I hope I am making sense, I tend to ramble.
I hope this information helps
Have a wonderful week