Some pieces have lovely fire inside, must be broken up to find these special attributes, some have low amounts of fire.
Obsidian is truth-enhancing.  A strongly protective stone, it forms a shield against negativity.  It blocks psychic attack and absorbs negative energies from the environment.  Obsidian draws out mental stress and tension.  It stimulates growth on all levels, urging exploration of the unknown and opening new horizons.  Brings clarity to the mind and clears confusion.  Helps you to know who you truly are.  Obsidian dissolves emotional blockages and ancient traumas.  Promotes qualities of compassion and strength.
Obsidian aids the digestion and detoxifies.  It reduces arthritis pain, joint problems and cramps.  Warms the extremities.