As for myself I like marble or granite. The trick is to get a thicker piece. Maybe 1 1/2' to 2" thick. Remember to get it onto a solid surface too. I jst got ahold of a black granite piece which is 18"x18" and 1 1/2" thick. Most of te time I have used a good thick slab of marble. It will make all the differance in your stamping.
Storm