As a general rule of thumb, I equate the black crystals with protection.
I am lucky enough to have a huge piece of obsidian in my home. It is currently in my family room. I find that an obsidian ball draws me into its energy and keeps me grounded and connected. In addition, I find it helpful in absorbing negative energies and will provide support during massive changes.
It is associated with your base Chakra and the signs of Sagittarius and Scorpio.
