Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral gypsum (calcium sulfate), often supplied in sticks of compressed powder about 10 cm (4 in) long. Sidewalk chalk is similar to blackboard chalk, but it is shaped into larger sticks and often colored. It is used to draw on sidewalks, streets, and driveways. Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors. However, it is now usually made from talc (magnesium silicate). (source)