Hi
It sounds like it could be asbestos board. I would do three things: 1. Take a sample and send it to be analized. If it is asbestos, check with your state government to see if you can remove it yourself or if you have to hire a liscensed contractor. 2. Learn the precautions to take while working with it (keep it wet, keep breakage to a minimum, wear protective clothing etc.) 3. Make sure you know how to dispose of it. Where to take it and how to double bag it.
Jim
