Obsidian is a great stone to add to your collection.
It is molten lava that has cooled quickly. These tumbled stones are sizable and great to add in a meditation or area where you practice self reflection. They're also excellent for absorbing EMFs.
This stone is a protective stone and guards against any negativity. It is a grounding stone, relieves tension and brings mental clarity. This stone is the perfect stone when you need to face reality and clean up areas of your past.