Not posting because I want a cube (I'm already a Magic Chalk user), because all 25 posts are already in. But I just want to say to anyone that hasn't tried Russian Magic Chalk -- you need to!
It's simply the best chalk out there. I've been using it for years. It's hard to find, but well worth the effort. I use cubes and wear 'em flat ("X"-pattern chalking, no drilling) until they are only 1/4-inch in height.