What do teachers use to write on blackboards?

Did someone say Chalk? If you did you're absolutely wrong!

   School used 'chalk' is not actually chalk, it's gypsum. Chalk is made of calcium carbonate ( as is limestone, marble, human and fish bones, eye lenses, indigestion pills and the limescale in kettles).
   Gypsum is made of calcium sulphate, they might look very much similar, but they aren't even made of the same chemical elements! Many substances that appear to be inherently different are actually made up of exactly the same chemical elements. Take the combination of carbon, hydrogen and oxygen for example. Synthesised in different ad-measurement they make radically different stuff such as aspirin, cholesterol, testosterone, alcohol, vinegar and glucose.
   Gypsum (scientifically called hydrated calcium sulphate) is one of the most widely abundant minerals in the world, being mined for more than 4,000 years. The plasterwork inside the ancient egyptian pyramids is made of the substance, as well as its use today in a large variety of industrial construction (most commonly used in ordinary building plaster of today).

   Nearly 3/4 of all gypsum is used in plaster and also in products such as tiles, plasterboard and 'plaster of Paris'. It's a key ingredient in the produce of cement and also used in paper and textiles. The average American home contains over 7 Tons of Gypsum.
   'Plater of Paris' is so called because the clay soil in and around Paris is packed with gypsum, most concentrated in Montmartre.

   Alabaster is the natural form of Gypsum. It's a snow-white, translucent material used in statues, busts and vases. It can be artificially dyed any colour and also may resemble marble when heated. Powdered alabaster is believed to help one centre themselves and also promote mental acuity (ability to reason). It was common for people to chip off pieces of church statues to make an ointment for bad legs as well.
   What's so ironic is is that the word 'gypsum' is from the Greek word gypsos, which means 'chalk'