Blackboard ‘chalk’ are produced especially for use with blackboards in white and also in various colours. White chalk sticks are made mainly from calcium carbonate derived from mineral chalk rock or limestone, while colored or pastel chalks are made from calcium sulfate in its dihydrate form, CaSO·2HO, derived from gypsum. Chalk sticks containing calcium carbonate typically contain 40–60% of CaCO (calcite). As compared to whiteboards, blackboards still have a variety of advantages: On the other hand, chalk produces dust, the amount depending on the quality of chalk used. Some people find this uncomfortable or may be allergic to it, and according
Chalk outline fairy" is occasionally used to describe an officer that makes the chalk outline, often without authorization and while unwittingly contaminating the scene. While the use of chalk is uncommon, investigators may make smaller marks, or use removable flags, index cards, or even markers, to indicate important positions, particularly if other references are not available. Chalk outlines of bodies are familiar tropes in popular culture. Often they are used in humorous ways depicting awkward positioning or meticulous precision, or portending a character's impending death by having them prematurely fall into a drawn outline. "The Naked Gun" series made extensive use of