Obsidian

Although I have a photo of a back Obsidian stone you can also have ones of other colours, I also have a Snowflake Obsidian which is black with white flecks resembling snowflakes giving the obvious name to that particular stone.

The Black Obsidian is very black, you feel you are drawn deeply into the stone while looking at it, and it is used, in fact, often as a mirror stone because it reflects back at you. You need to be careful as it is said to reflect back at you your own flaws, but this is counted as a good thing, it means you can work on fixing them once you see them clearly. It was used a scrying stone by the ancients due to it's mirror reflectiveness, too.

But it is a strong protective stone, too. It'll draw away negative energies, either from around you or from yourself, taking away any tensions and mental stress you might have. Meditate on it to draw them out and it'll send those negative energies into the ground. The stone will clear confusion and help draw out any forgotten abilities you had.

Obsidian will open you up to new ideas and show you new horizons to strive towards. Feng Shui says you should place a Black Obsidian in the North, or Bagua, area of your home the area connected with career and your path in life to allow it to clear obstacles in your way to fulfilling ambition in this area.

Pain is alleviated with this stone, from disorders such as arthritis and joint pain, you can do this by placing a stone against the area affected and wrapping a bandage around it to hold it there. If you have bad circulation this stone will help with that too. If you find it too forceful to use you can temper this by also using a more gentle stone  with it such as Rose Quartz, or even using the Snowflake Obsidian which does the same things as a Black one but is more soft for users.

An interesting extra use, the colour black is said to be an absence of light and this stone is meant to help make the user invisible - I bet Harry Potter would have like one of these.