Inexpensive and easy to obtain, chalk is used as a spiritual tool in many cultures. In Christianity, it is often used for the blessing of the Epiphany. In Hinduism auspicious designs are drawn on the doorstep at the beginning of the day. Essential to Hoodoo practitioners, chalk is used in Foot Track magick in which symbols are placed in the path of the spell's intended. In modern pagan craft, chalk is used much the same way. Made of calcium sulfate or calcium carbonate, chalk can represent the Earth aspect of most basic spells. When casting a small circle, chalk can be used as the circle's physical manifestation. For blessings and protection spells, it can be used to mark corresponding symbols on most surfaces with little fear to damaging property.
A box of basic white classroom chalk is all that is really needed in discreet witchery. For large-scale blessings (say...you entire driveway) children's sidewalk chalk is better. The sidewalk chalk is softer and the colors bolder. In a pinch, the sidewalk chalk can be used to write on skin for body blessings or protection spells, but it will rub off easily on clothing.
Bless the chalk as you would any of your magickal tools, preferably with smoke. Charge it under the full moon for extra *umph* (though be sure to bring it in before the dew settles--wet chalk is a messy thing). For magick on the go, nothing beats chalk. It fits handily in purse or pocket and it's uses are only limited by the witch's imagination.