Chalk
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Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed under the sea by the accumulation and lithification of hard
Beachy Head is a part of the extensive Southern England Chalk Formation.
"Nitzana Chalk curves" situated at Western Negev, Israel, are chalk deposits formed in the Mesozoic era's Tethys Ocean
Open chalk pit, Seale, Surrey, UK
Chalk from the White Cliffs of Dover, England
Blackboard
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A blackboard or a chalkboard is a reusable writing surface on which text or drawings are made with sticks of calcium sulphate or calcium carbonate, better known as chalk.
Chalk
…use in industry, such as for quicklime, bricks and builder's putty, and in agriculture, for raising pH in soils with high acidity. It is also used for "blackboard chalk" for writing and drawing on various types of surfaces, although these can also be manufactured from other carbonate-based minerals or gypsum. Chalk is…
NASA scientist Homer E. Newell Jr. explaining principles of altitude, pressure, and temperature, c. 1973
A chalkboard with an eraser in the chalk tray
Stick-slip effect with a chalk on a blackboard
Retouched drawing of a teacher at blackboard for an advertisement, 1924