Imagination Starters Chalkboard Crayons.  These are have a unique formula that is non-toxic, washable and wipes off with just water.  These work great on our placemats and flash cards and even glass!  They do also work on non-porous chalkboard but since no one ever knows if their board is, use caution and test in a corner to be safe.

Colors included: Red, White, Blue and Yellow