Obsidian

Black Obsidian is an extremely powerful and fast acting crystal and should be used with caution for crystal healing. It can bring negative emotions and unpleasant truths to the surface, which need confronting before peace can be restored. It clears subconscious blocks, but should not be used unconsciously.
Black obsidian is a protective stone and is said to form a shield against negativity and absorb negative energies from the environment and because of this it needs to be cleansed regularly under running water.
Place Black Obsidian on the base chakra for grounding.
In healing it is said to reduce the pain of arthritis, help joint problems and ease cramp. Also believed to aid digestion.