Yes, black stones...
And also mahogany obsidian, which is a reddish black. I like snowflake obsidian as well, although I feel it's 'softer' and less militant in it's protection.
I use hematite to absorb negativity, both mine and others'. As a result, I cleanse my hematite often in salt water or under the full moon. There is also a space clearing spray called "Crystal Clear", by Petaltone essences (it's based in the UK) that is supposed to work wonderfully well for cleaning away old energies from both crystals and places in general.
I think that what Sulis is talking about is a visualized mirrored psychic shield. A huge part of magic is visualization, and that requires some mental discipline. Basically, a mesh-like psychic shield would serve like armor, while a mirror-like one would actually deflect the negativity. If you can see it in your mind's eye, and keep it there in the back of your mind throughout your day, it will work for you.
There are many books on psychic protection out there, but one that you may find useful is by Judy Hall (a Brit, I believe) entitled The Art of Psychic Protection. Plainly written and easy to work with, without a lot of the extraneous issues that some people believe in and others do not.