Chalk is a white powder made of soft limestone called calcium carbonate. It can be found in both a powder substance or a more firm rock-like form. One use of the powder form is for gymnasts. They use the calcium carbonate powder from to protect their hands from blisters when doing work on the bars, pomel horse, or rings. The harder form of calcium carbonate can sometimes be found in the shape of a long round cylinder and is used as a writing implement on blackboards. Sometimes it is mixed with a coloring agent and used to draw pictures on sidewalks.