From what is made the chalk?

Have you ever as a child eaten a piece of chalk?
Have you ever think from what is the chalk made?

The chalk

    The chalk is a type of sedimentary rock. Chemically, it is similar to the limestone, but the chalk consists more than 90% of CaC03 (or calcium carbohydrate) Therefore, it's very pure.
    The chalk was laid down in the Upper Cretaceous period. It was created from little calcite shelves or skeletons of tiny sea microorganisms (plankton) called cocoliths

Physical characteristics: The chalk is very friable, porous and weekly reinforced.
It has white or gray color.
In comparison with the clay, the chalk is more likely resistant to sliding, and therefore often creates steep cliffs near the sea.

I think that most widely recognized are chalk cliffs In Ireland or Southern England (this pic is from Studland, Dorset)


But actually to make the chalk like we all know it:

there is a need for a little chemical alternation:


The blackboard chalk, or commonly known as chalkboard, is actually gypsum ( CaSO4 or calcium sulfate)
Its unique physical trait of softness allows us to write with it on hard surfaces - it will leave a mark.

I think there's often a misused term for the chalkboard, because it actually differs from the substance of chalk cliffs.

Hope this helps:)


Sources: 
http://cs.wikipedia.org/wiki/K%C5%99%C3%ADda_%28hornina%29
http://gwydir.demon.co.uk/jo/minerals/chalk.htm
http://www.gsi.ie/Education/European+Landscapes/United+Kingdom.htm
Czech Book: Horniny z doby křídy (written by Slátal 1998)